fix(web): handle unhandled promise rejection in CancellableTask (#27553)

When a concurrent caller awaits `this.complete` inside `execute()` and
`cancel()` is called, the promise rejects with `undefined` outside of any
try/catch, causing "Uncaught (in promise) undefined" console spam during
rapid timeline scrolling.

- Wrap the `await this.complete` path in try/catch, returning 'CANCELED'
- Guard the `finally` block to only null `cancelToken` if it still belongs
  to this call, preventing a race condition with `cancel()` to `init()`

Change-Id: I65764dd664eb408433fc6e5fc2be4df56a6a6964

web/src/lib/utils/cancellable-task.ts

@@ -64,8 +64,12 @@ export class CancellableTask {
       if (this.cancellable && !cancellable) {
         this.cancellable = cancellable;
       }
-      await this.complete;
-      return 'WAITED';
+      try {
+        await this.complete;
+        return 'WAITED';
+      } catch {
+        return 'CANCELED';
+      }
     }
     this.cancellable = cancellable;
     const cancelToken = (this.cancelToken = new AbortController());
@@ -86,7 +90,9 @@ export class CancellableTask {
       this.#transitionToErrored(error);
       return 'ERRORED';
     } finally {
-      this.cancelToken = null;
+      if (this.cancelToken === cancelToken) {
+        this.cancelToken = null;
+      }
     }
   }