fix swallowing errors

Author: shirley-perplexity

src/mcp/client/session.py

@@ -57,6 +57,9 @@ async def __call__(
 async def _default_message_handler(
     message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
 ) -> None:
+    if isinstance(message, Exception):
+        # Re-raise exceptions so they propagate back to the application
+        raise message
     await anyio.lowlevel.checkpoint()