fix(distributed): set node header before first byte on streaming error path

In the chat.go error-from-worker branch the call to applyNodeIDHeader
came after fmt.Fprintf had already written to the response writer.
Go's http.ResponseWriter commits headers on the first Write, so the
X-LocalAI-Node header was silently dropped on streaming error
responses. Move the call before each Fprintf so the header is set on
both the marshal-error and the normal-error paths.

The non-streaming chat path and the completion error path were
already correctly ordered.

Assisted-by: Claude:claude-opus-4-7[1m]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

core/http/endpoints/openai/chat.go

@@ -449,12 +449,13 @@ func ChatEndpoint(cl *config.ModelConfigLoader, ml *model.ModelLoader, evaluator
 						respData, marshalErr := json.Marshal(errorResp)
 						if marshalErr != nil {
 							xlog.Error("Failed to marshal error response", "error", marshalErr)
+							applyNodeIDHeader()
 							fmt.Fprintf(c.Response().Writer, "data: {\"error\":{\"message\":\"Internal error\",\"type\":\"server_error\"}}\n\n")
 						} else {
+							applyNodeIDHeader()
 							fmt.Fprintf(c.Response().Writer, "data: %s\n\n", respData)
 						}
 						fmt.Fprintf(c.Response().Writer, "data: [DONE]\n\n")
-						applyNodeIDHeader()
 						c.Response().Flush()
 
 						return nil