fix(tests): add pragma no branch to fix coverage false positive

The suppressing_handler in test_custom_message_handler_can_suppress_exceptions
only ever receives Exception messages in the test, so coverage.py reports
the False branch (792->exit) as uncovered. This is a known coverage.py
quirk with async functions where one branch path is never taken by design.

Adding # pragma: no branch restores 100% coverage.

Github-Issue: #1401

Author: Aboudjem

tests/client/test_session.py

@@ -789,7 +789,7 @@ async def test_custom_message_handler_can_suppress_exceptions():
     async def suppressing_handler(
         message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
     ) -> None:
-        if isinstance(message, Exception):
+        if isinstance(message, Exception):  # pragma: no branch
             suppressed_exceptions.append(message)
             # Intentionally NOT re-raising — old silent behavior