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.spec.ts

@@ -161,6 +161,30 @@ describe('CancellableTask', () => {
       expect(task.executed).toBe(true);
     });
 
+    it('should return CANCELED when concurrent caller is waiting and task is canceled', async () => {
+      const task = new CancellableTask();
+      let resolveTask: () => void;
+      const taskPromise = new Promise<void>((resolve) => {
+        resolveTask = resolve;
+      });
+      const taskFn = async (signal: AbortSignal) => {
+        await taskPromise;
+        if (signal.aborted) {
+          throw new DOMException('Aborted', 'AbortError');
+        }
+      };
+
+      const promise1 = task.execute(taskFn, true);
+      const promise2 = task.execute(taskFn, true);
+
+      task.cancel();
+      resolveTask!();
+
+      const [result1, result2] = await Promise.all([promise1, promise2]);
+      expect(result1).toBe('CANCELED');
+      expect(result2).toBe('CANCELED');
+    });
+
     it('should not cancel if task is already executed', async () => {
       const task = new CancellableTask();
       const taskFn = vi.fn(async () => {});

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;
+      }
     }
   }