fix(failover): send remote targets their own upstream model

A chain request reached a cloud-proxy target with the client's model,
the chain name, whenever the target set no upstream_model: passthrough
forwards the body's model and translate falls back to it. The upstream
answered 404, which neither retries nor trips, while the liveness
probe, which checks the target's own name, kept passing.

PrepareTarget now sets the upstream model of a remote target to
proxy.upstream_model or the target name, the same name the probe uses.
The request pipeline and realtime chain stages both call it.

Assisted-by: Claude:claude-opus-5-5

tests/e2e/e2e_failover_test.go

@@ -106,6 +106,11 @@ var _ = Describe("Failover chains", Label("failover"), func() {
 			Expect(resp.Header.Get("X-LocalAI-Served-Model")).To(Equal("up-2"))
 			Expect(resp.Header.Get("X-LocalAI-Failover")).To(Equal("fallback"))
 			Expect(chainActive("chain-remote")).To(Equal("up-2"))
+			// The targets set no upstream_model: each upstream must get its
+			// target's name (what the liveness probe checks), not the chain
+			// name the client sent. up-2 is healthy, so only this request
+			// posted to it.
+			Expect(upstreamBodyModel(up2)).To(Equal("up-2"))
 
 			up1.SetScript(chatReply)
 			Eventually(func() string { return chainActive("chain-remote") }, 30*time.Second, 500*time.Millisecond).
@@ -116,10 +121,22 @@ var _ = Describe("Failover chains", Label("failover"), func() {
 			Expect(resp2.StatusCode).To(Equal(200))
 			Expect(resp2.Header.Get("X-LocalAI-Served-Model")).To(Equal("up-1"))
 			Expect(resp2.Header.Get("X-LocalAI-Failover")).To(BeEmpty())
+			Expect(upstreamBodyModel(up1)).To(Equal("up-1"))
 		})
 	})
 })
 
+// upstreamBodyModel returns the "model" field of the last request body the
+// fake upstream recorded.
+func upstreamBodyModel(up *fakeOpenAIUpstreamServer) string {
+	_, _, _, body := up.recorder.snapshot()
+	var req struct {
+		Model string `json:"model"`
+	}
+	Expect(json.Unmarshal(body, &req)).To(Succeed(), string(body))
+	return req.Model
+}
+
 // chainActive returns the active target of a chain as the REST status reports
 // it, or "" when the status cannot be read.
 func chainActive(chain string) string {