fix(openresponses): parse OpenAI-spec nested tool_choice + use correct setter (#9509)

Two bugs in MergeOpenResponsesConfig (/v1/responses + WebSocket, *not*
/v1/chat/completions — that has a separate, working path via Tool
unmarshal + SetFunctionCallNameString):

1. **Shape mismatch.** OpenAI's specific-function tool_choice nests the
   name under "function":
       {"type": "function", "function": {"name": "my_function"}}
   The legacy flat shape was:
       {"type": "function", "name": "my_function"}
   Only the flat shape was handled. OpenAI-compliant clients that reach
   /v1/responses (openai-python with the Responses API, Stainless-generated
   SDKs, …) silently failed to force the function.

2. **Wrong setter.** The code called SetFunctionCallString(name), which
   writes the mode field (functionCallString: "none"/"auto"/"required").
   The specific-function name lives in a separate field
   (functionCallNameString), read by ShouldCallSpecificFunction and
   FunctionToCall. Net effect: a correctly-formed tool_choice never
   engaged grammar-based forcing.

The fix preserves backward compatibility by accepting both shapes
(nested preferred, flat as fallback) and routes to the correct setter.

Note: The same "wrong setter" pattern appears at three other sites —
anthropic/messages.go:883, openai/realtime_model.go:171, and
openresponses/responses.go:776 — and /v1/chat/completions has its own
issue parsing tool_choice="required" as a string (json.Unmarshal on a
raw string fails silently). Those are filed as a tracking issue rather
than bundled here to keep this PR focused.

## Test plan
9 new Ginkgo specs under "MergeOpenResponsesConfig tool_choice parsing":
  - string modes: "required" / "auto" / "none"
  - OpenAI-spec nested shape: {type:function, function:{name}}
  - Legacy Anthropic-compat flat shape: {type:function, name}
  - Shape-preference: nested wins over flat when both present
  - Malformed: missing type, wrong type, missing name, empty name, nil

$ go test ./core/http/middleware/ -count=1 -run TestMiddleware
  Ran 28 of 28 Specs in 0.003 seconds -- PASS

## Repro (against /v1/responses)

    curl -N http://localai/v1/responses \
         -H 'Content-Type: application/json' \
         -d '{"model":"qwen3.6-35b-a3b-apex",
              "input":"Weather in Berlin?",
              "tools":[{"type":"function","name":"get_weather",
                        "parameters":{"type":"object",
                          "properties":{"city":{"type":"string"}},
                          "required":["city"]}}],
              "tool_choice":{"type":"function",
                             "function":{"name":"get_weather"}}}'

Before: grammar-based forcing silently inactive; model free-texts.
After : grammar forces get_weather invocation; output contains
        tool_calls with function:{name:"get_weather", arguments:{...}}.

core/http/middleware/request.go

@@ -614,10 +614,31 @@ func MergeOpenResponsesConfig(config *config.ModelConfig, input *schema.OpenResp
 			}
 			// "auto" is default - let model decide
 		case map[string]any:
-			// Specific tool: {type:"function", name:"..."}
+			// Specific tool. OpenAI spec nests the function name under "function":
+			//   {"type":"function", "function":{"name":"..."}}
+			// Legacy/Anthropic-compat form puts it at the top level:
+			//   {"type":"function", "name":"..."}
+			// The old code only handled the legacy shape AND used the wrong
+			// setter (SetFunctionCallString writes the mode field; the
+			// specific-function name lives in a separate field read by
+			// ShouldCallSpecificFunction / FunctionToCall). Net effect: a
+			// correctly-formed OpenAI tool_choice never engaged grammar-based
+			// forcing, the model got the tools but no selection hint, and
+			// streamed raw JSON as delta.content instead of delta.tool_calls.
 			if tcType, ok := tc["type"].(string); ok && tcType == "function" {
-				if name, ok := tc["name"].(string); ok {
-					config.SetFunctionCallString(name)
+				var name string
+				if fn, ok := tc["function"].(map[string]any); ok {
+					if n, ok := fn["name"].(string); ok {
+						name = n
+					}
+				}
+				if name == "" {
+					if n, ok := tc["name"].(string); ok {
+						name = n
+					}
+				}
+				if name != "" {
+					config.SetFunctionCallNameString(name)
 				}
 			}
 		}

core/http/middleware/request_test.go

@@ -150,3 +150,159 @@ var _ = Describe("SetModelAndConfig middleware", func() {
 		})
 	})
 })
+
+// ---------------------------------------------------------------------------
+// MergeOpenResponsesConfig — tool_choice parsing
+// ---------------------------------------------------------------------------
+//
+// The OpenAI chat/completions spec nests the function name under "function":
+//     {"type":"function", "function":{"name":"my_function"}}
+// The legacy Anthropic-compat shape puts it at the top level:
+//     {"type":"function", "name":"my_function"}
+// Both need to reach SetFunctionCallNameString (not SetFunctionCallString,
+// which is the mode field "none"/"auto"/"required").
+//
+// These specs assert both shapes populate the specific-function name and that
+// downstream predicates (ShouldCallSpecificFunction, FunctionToCall) return
+// the expected values so grammar-based forcing actually engages.
+var _ = Describe("MergeOpenResponsesConfig tool_choice parsing", func() {
+	var cfg *config.ModelConfig
+
+	BeforeEach(func() {
+		cfg = &config.ModelConfig{}
+	})
+
+	Context("string tool_choice", func() {
+		It("sets mode to required for tool_choice=\"required\"", func() {
+			req := &schema.OpenResponsesRequest{ToolChoice: "required"}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			// "required" is a mode, not a specific function.
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeFalse())
+			// ShouldUseFunctions must be true so tools are sent to the model.
+			Expect(cfg.ShouldUseFunctions()).To(BeTrue())
+		})
+
+		It("leaves config untouched for tool_choice=\"auto\"", func() {
+			req := &schema.OpenResponsesRequest{ToolChoice: "auto"}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeFalse())
+			Expect(cfg.FunctionToCall()).To(Equal(""))
+		})
+
+		It("leaves config untouched for tool_choice=\"none\"", func() {
+			req := &schema.OpenResponsesRequest{ToolChoice: "none"}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeFalse())
+			Expect(cfg.FunctionToCall()).To(Equal(""))
+		})
+	})
+
+	Context("specific-function tool_choice (OpenAI spec shape)", func() {
+		It("parses {type:function, function:{name:...}} and sets the specific-function name", func() {
+			req := &schema.OpenResponsesRequest{
+				ToolChoice: map[string]any{
+					"type":     "function",
+					"function": map[string]any{"name": "get_weather"},
+				},
+			}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			// This is the key invariant the fix restores: a correctly-formed
+			// OpenAI tool_choice must result in ShouldCallSpecificFunction()=true.
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeTrue())
+			Expect(cfg.FunctionToCall()).To(Equal("get_weather"))
+		})
+
+		It("prefers the nested function.name over a stray top-level name", func() {
+			// Defense-in-depth: both shapes present, OpenAI spec wins.
+			req := &schema.OpenResponsesRequest{
+				ToolChoice: map[string]any{
+					"type":     "function",
+					"function": map[string]any{"name": "correct_name"},
+					"name":     "legacy_name",
+				},
+			}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			Expect(cfg.FunctionToCall()).To(Equal("correct_name"))
+		})
+	})
+
+	Context("specific-function tool_choice (legacy Anthropic-compat shape)", func() {
+		It("parses {type:function, name:...} and sets the specific-function name", func() {
+			req := &schema.OpenResponsesRequest{
+				ToolChoice: map[string]any{
+					"type": "function",
+					"name": "get_weather",
+				},
+			}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeTrue())
+			Expect(cfg.FunctionToCall()).To(Equal("get_weather"))
+		})
+	})
+
+	Context("malformed tool_choice", func() {
+		It("is a no-op when type is missing", func() {
+			req := &schema.OpenResponsesRequest{
+				ToolChoice: map[string]any{
+					"function": map[string]any{"name": "get_weather"},
+				},
+			}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeFalse())
+		})
+
+		It("is a no-op when type is not \"function\"", func() {
+			req := &schema.OpenResponsesRequest{
+				ToolChoice: map[string]any{
+					"type":     "object",
+					"function": map[string]any{"name": "get_weather"},
+				},
+			}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeFalse())
+		})
+
+		It("is a no-op when name is missing from both shapes", func() {
+			req := &schema.OpenResponsesRequest{
+				ToolChoice: map[string]any{
+					"type":     "function",
+					"function": map[string]any{},
+				},
+			}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeFalse())
+			Expect(cfg.FunctionToCall()).To(Equal(""))
+		})
+
+		It("is a no-op when name is empty string", func() {
+			req := &schema.OpenResponsesRequest{
+				ToolChoice: map[string]any{
+					"type":     "function",
+					"function": map[string]any{"name": ""},
+				},
+			}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeFalse())
+		})
+	})
+
+	Context("nil tool_choice", func() {
+		It("is a no-op", func() {
+			req := &schema.OpenResponsesRequest{ToolChoice: nil}
+			Expect(MergeOpenResponsesConfig(cfg, req)).To(Succeed())
+
+			Expect(cfg.ShouldCallSpecificFunction()).To(BeFalse())
+			Expect(cfg.FunctionToCall()).To(Equal(""))
+		})
+	})
+})