fix(core): threadsafe asyncio future cancellation in CancellationToken

[MOHAMMED WASIM KHAN (wasim-builds)]

Closes #8358

Summary

Fixes cross-thread cancellation in CancellationToken.link_future() when cancellation originates from a background thread or thread pool worker.

Problem

Previously, CancellationToken.cancel() executed callbacks synchronously in the invoking thread, calling future.cancel() directly outside the future's owning event loop. Under debug mode or cross-thread execution, this raised:
RuntimeError: Non-thread-safe operation invoked on an event loop other than the current one

Solution

• Introduced threadsafe future cancellation via loop.call_soon_threadsafe(future.cancel) when the future's owning event loop is running.
• Falls back to direct cancellation when the loop is not running or shutdown.
• Added unit test test_cancellation_token_threadsafe_link_future verifying safe future cancellation from worker threads.

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