test: fix promise-watcher test unhandled rejections in Ava 6

The test intentionally creates unhandled rejections to validate promise watcher behavior. Added handler to prevent Ava 6 from killing the process when these expected unhandled rejections occur.

Co-authored-by: turadg <21505+turadg@users.noreply.github.com>

Author: Copilot

packages/SwingSet/test/promise-watcher/unhandledRejectionDetector.js

@@ -6,4 +6,10 @@ export function handleUnhandledRejections(handler) {
   unhandledRejectionHandler = handler;
 }
 
-process.on('unhandledRejection', rej => unhandledRejectionHandler(rej));
+process.on('unhandledRejection', rej => {
+  if (unhandledRejectionHandler) {
+    unhandledRejectionHandler(rej);
+  }
+  // Prevent Ava 6 from exiting the process on unhandled rejection
+  // when we're intentionally testing unhandled rejections
+});