feat(activity): notify Activities when the Worker starts shutting down

[rohitsudhakar1]

Closes #1739.

What #1739 asked for

workerShuttingDown: Promise<never> and workerShuttingDownSignal: AbortSignal on the Activity Context, mirroring how cancellation is handled, plus matching top-level exports from activity/index.ts.

What this implements

• Context.workerShuttingDown — a Promise<never> that rejects with CancelledFailure('WORKER_SHUTDOWN') as soon as the Worker initiates shutdown, and Context.workerShuttingDownSignal — the corresponding AbortSignal. These mirror the existing cancelled / cancellationSignal pair.
• Top-level exports workerShuttingDown() and workerShuttingDownSignal() from @temporalio/activity, as shortcuts for Context.current().…, consistent with the existing cancelled() / cancellationSignal() helpers.
• The Worker owns a single AbortController, aborted from the state setter as soon as the Worker leaves RUNNING, so every shutdown path is covered — including error-driven ones — rather than just the happy-path shutdown() call. Each Activity derives its own controller from that shared signal, so user code can register listeners without accumulating them on the Worker's signal for the Worker's whole lifetime.
• A shutdown notification is deliberately not a cancellation: cancellationSignal and cancellationDetails are left untouched and the Activity remains free to complete normally. Unlike cancellation, an Activity also does not need to heartbeat() to learn about shutdown.
• MockActivityEnvironment.notifyWorkerShuttingDown(), so Activities that use the new API are testable without standing up a real Worker. This wasn't in the issue text, but the API is untestable in user code without it.

Both Context members are documented as @experimental.

Tests

Four new tests in packages/testing/src/__tests__/test-mockactivityenv.ts:

1. the notification resolves workerShuttingDown and aborts workerShuttingDownSignal, and the Activity still completes normally afterwards;
2. it does not cancel the Activity — cancellationSignal stays un-aborted, cancellationDetails() stays undefined, and sleep() still works;
3. the idiomatic Promise.race([work, workerShuttingDown()]) bail-out pattern;
4. the notification is idempotent and applies to Activities started after shutdown was initiated.

These provably fail without the change: reverting only the four implementation files and rebuilding fails compilation on every new API reference (TS2339: Property 'workerShuttingDown' does not exist…), so the tests cannot pass against main. With the change, the full test-mockactivityenv suite passes (8/8). eslint, prettier --check and lint:prune are all clean.

Cross-SDK parity

This brings TypeScript in line with the other SDKs, which all expose worker-shutdown notification distinctly from cancellation:

SDK	API
Go	activity.GetWorkerStopChannel
Python	activity.wait_for_worker_shutdown
.NET	ActivityExecutionContext.WorkerShutdownToken

The Promise / AbortSignal pairing follows the shape #1739 specified and matches how this SDK already surfaces cancellation, rather than importing another SDK's idiom.

Happy to adjust naming, the @experimental markers, or the choice to fire on any non-RUNNING state transition if maintainers would prefer it scoped only to explicit shutdown().

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