fix: use model_validate to avoid pyright reportArgumentType error

Replace **dict unpacking with model_validate() in the extra fields test.
Pyright cannot narrow dict value types through ** unpacking, causing
false reportArgumentType errors for typed params (task, _meta).

Author: Br1an67

tests/test_types.py

@@ -372,7 +372,7 @@ def test_request_params_allows_extra_fields():
     """
     from mcp.types import RequestParams
 
-    params = RequestParams(**{"custom_field": "value", "another": 42})
+    params = RequestParams.model_validate({"custom_field": "value", "another": 42})
     assert params.model_extra == {"custom_field": "value", "another": 42}
 
     # Extra fields survive serialization round-trip