fix(test): update message_handler to the current IncomingMessage type

RequestResponder and mcp.shared.session were removed upstream since
this test was written; message_handler callbacks now receive
IncomingMessage (ServerNotification | Exception), matching every
other message_handler in this file.

Author: STiFLeR7

tests/client/test_session.py

@@ -205,9 +205,7 @@ async def mock_server():
             )
 
     # Create a message handler to catch exceptions
-    async def message_handler(  # pragma: no cover
-        message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
-    ) -> None:
+    async def message_handler(message: IncomingMessage) -> None:  # pragma: no cover
         if isinstance(message, Exception):
             raise message