fix(examples/chat): import vitest globals in e2e-overrides spec (unbreak app build) (#679)

`tsconfig.app.json` includes `src/**/*.ts` with `types: []`, so the Angular
app build type-checks every spec. `e2e-overrides.spec.ts` (added in #677) used
ambient `describe`/`it`/`expect`/`afterEach`, which fail under the app build
(`TS2304`/`TS2593`) — the bundle never generates, the e2e server never starts,
and all 4 `examples/chat` e2e shards time out. Matches the existing specs by
importing the globals from vitest explicitly.

Regressed main at 3c45540 (#677 merge); local `nx test` masked it because only
`nx serve`/the app build compiles specs under tsconfig.app.json.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: blove

examples/chat/angular/src/app/shell/e2e-overrides.spec.ts

@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: MIT
+import { describe, it, expect, afterEach } from 'vitest';
 import { e2eClientOptions } from './e2e-overrides';
 
 const KEY = 'THREADPLANE_E2E_MAX_RETRIES';