Log warning for unhandled exceptions in default message handler

Transport errors that are not tied to a specific pending request (e.g.,
GET stream failures) were silently swallowed by the default message
handler. Add a warning log so these exceptions are at least visible in
logs as an observability safety net.

Github-Issue: #1401

Author: heyhayes

src/mcp/client/session.py

@@ -57,6 +57,8 @@ async def __call__(
 async def _default_message_handler(
     message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
 ) -> None:
+    if isinstance(message, Exception):
+        logger.warning("Unhandled exception in message handler: %s", message)
     await anyio.lowlevel.checkpoint()