fix: use Responses-compatible function tool_choice format

backend/internal/pkg/apicompat/chatcompletions_to_responses.go

@@ -420,7 +420,7 @@ func convertChatToolsToResponses(tools []ChatTool, functions []ChatFunction) []R
 //
 //	"auto" → "auto"
 //	"none" → "none"
-//	{"name":"X"} → {"type":"function","function":{"name":"X"}}
+//	{"name":"X"} → {"type":"function","name":"X"}
 func convertChatFunctionCallToToolChoice(raw json.RawMessage) (json.RawMessage, error) {
 	// Try string first ("auto", "none", etc.) — pass through as-is.
 	var s string
@@ -436,7 +436,7 @@ func convertChatFunctionCallToToolChoice(raw json.RawMessage) (json.RawMessage,
 		return nil, err
 	}
 	return json.Marshal(map[string]any{
-		"type":     "function",
-		"function": map[string]string{"name": obj.Name},
+		"type": "function",
+		"name": obj.Name,
 	})
 }