[Feature] TabsController and tab tools should honor AbortSignal

[gaomeng1900]

Context

Follow-up from the robust-aborting work in #532.

The extension's tab tools (open_new_tab, switch_to_tab, close_tab) and TabsController are not integrated with the per-invocation ctx.signal cancellation model. In particular, TabsController.waitUntilTabLoaded() polls via waitUntil() (up to a few seconds, default 60s elsewhere) with raw setTimeout and cannot be interrupted when the task is stopped or disposed.

Goal

• Pass the task AbortSignal into extension tab tool execution (packages/extension/src/agent/tabTools.ts).
• Make TabsController.waitUntilTabLoaded() and the underlying waitUntil() accept { signal } so first-party tab operations do not outlive stop/dispose.
• For remote content-script / background calls that cannot be truly cancelled, add a run-id or late-result guard so stale results cannot mutate the active run.

Notes

• Relevant files: packages/extension/src/agent/tabTools.ts, packages/extension/src/agent/TabsController.ts, packages/extension/src/agent/MultiPageAgent.ts.
• The cancellation contract is described in feat: robust aborting #532.

Planned in #532.