fix(grpc): support signal shutdown on Windows - #8082

Why are these changes needed?

GrpcWorkerAgentRuntime.stop_when_signal() currently calls loop.add_signal_handler() unconditionally. The default Windows event loop does not implement that API, so cross-language workers fail with NotImplementedError instead of waiting for shutdown. The gRPC host has the same signal-registration path.

This change keeps native asyncio signal handlers on supported event loops and falls back to signal.signal() when the event-loop API is unavailable. The fallback schedules shutdown on the owning loop with call_soon_threadsafe, and both paths remove or restore their handlers when waiting finishes. A shared helper keeps worker and host behavior consistent.

Related issue number

Closes #5760

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. No public API or documentation changes are required.
• I've added tests corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed. CI is pending.

Local validation:

• pytest -q python/packages/autogen-ext/tests/test_grpc_signal_utils.py — 2 passed on Windows
• Ruff 0.4.8 format and lint — passed for all four changed files
• Pyright 1.1.389 — 0 errors for all four changed files
• strict mypy — 0 errors for the new signal helper

AI assistance: Codex was used to inspect the failing paths, implement the fix, and run the checks above. The final diff and test behavior were reviewed before submission.