Fix tests for Pydantic 2.11 (modelcontextprotocol#465)

Author: medaminezghal

src/mcp/server/fastmcp/utilities/func_metadata.py

@@ -27,7 +27,7 @@ def model_dump_one_level(self) -> dict[str, Any]:
         That is, sub-models etc are not dumped - they are kept as pydantic models.
         """
         kwargs: dict[str, Any] = {}
-        for field_name in self.model_fields.keys():
+        for field_name in self.__class__.model_fields.keys():
             kwargs[field_name] = getattr(self, field_name)
         return kwargs