fix: define JSON type alias in TypedDictModelType imports

TypedDictModelType returns 'JSON' for response type annotations but
never defined the JSON = MutableMapping[str, Any] type alias, causing
NameError at runtime.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: iscai-msft

packages/http-client-python/generator/pygen/codegen/models/model_type.py

@@ -393,3 +393,8 @@ def docstring_text(self, **kwargs: Any) -> str:
         if kwargs.pop("is_response", False):
             return "JSON"
         return super().docstring_text(**kwargs)
+
+    def imports(self, **kwargs: Any) -> FileImport:
+        file_import = super().imports(**kwargs)
+        file_import.define_mutable_mapping_type()
+        return file_import