Add json_response.py comment

Author: praboud-ant

src/mcp/server/auth/json_response.py

@@ -4,5 +4,7 @@
 
 
 class PydanticJSONResponse(JSONResponse):
+    # use pydantic json serialization instead of the stock `json.dumps`,
+    # so that we can handle serializing pydantic models like AnyHttpUrl
     def render(self, content: Any) -> bytes:
         return content.model_dump_json(exclude_none=True).encode("utf-8")