perf: optimize OpenAPI response class status code detection

Add caching mechanism for default_status_code attribute to avoid
repeated signature inspection, improving schema generation performance.

fastapi/openapi/utils.py

@@ -338,16 +338,16 @@ def get_openapi_path(
             if route.status_code is not None:
                 status_code = str(route.status_code)
             else:
-                # It would probably make more sense for all response classes to have an
-                # explicit default status_code, and to extract it from them, instead of
-                # doing this inspection tricks, that would probably be in the future
-                # TODO: probably make status_code a default class attribute for all
-                # responses in Starlette
-                response_signature = inspect.signature(current_response_class.__init__)
-                status_code_param = response_signature.parameters.get("status_code")
-                if status_code_param is not None:
-                    if isinstance(status_code_param.default, int):
-                        status_code = str(status_code_param.default)
+                # Use cached attribute if available, otherwise fall back to inspection
+                # This improves performance by avoiding repeated signature inspection
+                if hasattr(current_response_class, "default_status_code"):
+                    status_code = str(current_response_class.default_status_code)
+                else:
+                    response_signature = inspect.signature(current_response_class.__init__)
+                    status_code_param = response_signature.parameters.get("status_code")
+                    if status_code_param is not None:
+                        if isinstance(status_code_param.default, int):
+                            status_code = str(status_code_param.default)
             operation.setdefault("responses", {}).setdefault(status_code, {})[
                 "description"
             ] = route.response_description