Update send_datagram_after_close test to expect ConnectionError

The test assertion now expects SessionError::ConnectionError(LocallyClosed)
instead of SessionError::SendDatagramError(ConnectionLost(LocallyClosed)),
matching the upcoming session error propagation behavior where connection-level
errors are replaced with the stored session error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ai-and-i

web-transport-browser-tests/tests/connection.rs

@@ -517,11 +517,9 @@ async fn server_send_datagram_after_close() {
             assert!(
                 matches!(
                     err,
-                    SessionError::SendDatagramError(quinn::SendDatagramError::ConnectionLost(
-                        quinn::ConnectionError::LocallyClosed
-                    ))
+                    SessionError::ConnectionError(quinn::ConnectionError::LocallyClosed)
                 ),
-                "expected SendDatagramError(ConnectionLost(LocallyClosed)), got {err:?}"
+                "expected ConnectionError(LocallyClosed), got {err:?}"
             );
         })
     });