quic: fix potential crash from unobserved closed

lib/internal/quic/quic.js

@@ -3586,7 +3586,9 @@ class QuicSession {
 
     if (error) {
       // If the session is still waiting to be closed, and error
-      // is specified, reject the closed promise.
+      // is specified, reject the closed promise. Mark it handled first
+      // (as with opened) to silence errors if it's not actually awaited.
+      markPromiseAsHandled(inner.pendingClose.promise);
       inner.pendingClose.reject?.(error);
     } else {
       inner.pendingClose.resolve?.();

test/parallel/test-quic-session-unobserved-error-close.mjs

@@ -0,0 +1,47 @@
+// Flags: --experimental-quic --no-warnings
+
+// Regression test: destroying a session with an error while its `closed`
+// promise is not being observed must NOT surface as an unhandled rejection.
+
+import { hasQuic, skip, mustCall, mustNotCall } from '../common/index.mjs';
+import { subscribe, unsubscribe } from 'node:diagnostics_channel';
+import { setImmediate as tick } from 'node:timers/promises';
+
+if (!hasQuic) {
+  skip('QUIC is not enabled');
+}
+
+const { listen, connect } = await import('../common/quic.mjs');
+
+// Any unhandled rejection - e.g. an unobserved `closed` rejecting - fails.
+process.on('unhandledRejection', mustNotCall('unexpected unhandled rejection'));
+
+// We use the diagnostics channel to observe the error close without actually
+// observing session.closed (not listening is the whole point of the test):
+const serverErrored = Promise.withResolvers();
+function onSessionError() {
+  unsubscribe('quic.session.error', onSessionError);
+  serverErrored.resolve();
+}
+subscribe('quic.session.error', onSessionError);
+
+// The server session callback is left deliberately empty, so no response
+// is sent and closed remains unobserved:
+const serverEndpoint = await listen(mustCall());
+
+const clientSession = await connect(serverEndpoint.address);
+await clientSession.opened;
+
+// Finish handshake before close:
+await tick();
+
+// Cleanly close from the client with an error code, so the server
+// receives a peer error close:
+await clientSession.close({ code: 1 });
+
+// Wait until the server has processed the error close, plus another tick
+// to ensure unobserved promise rejection doesn't fire anywhere:
+await serverErrored.promise;
+await tick();
+
+await serverEndpoint.close();