Deliver pending cancellation when an activity thread registers

[yannvr]

What changed

A synchronous activity can receive cancellation while waiting for an executor thread. The raiser stores the exception, but thread registration never delivers it, so the cancelled activity still executes. Registration now checks the pending exception under the existing lock and honours shielding.

Registration can now raise before the context manager yields, so it also moves inside the existing cleanup guard. The thread id is cleared on that path, keeping the executor thread available for the next activity.

Validation

• The new pre-start cancellation regression fails on unchanged main at fc5299b.
• Five focused tests cover pre-start cancellation, shielding, active cancellation and post-return thread reuse; all pass.
• Eight existing worker tests pass with the local dev server, including thread cancellation and shutdown.
• Broader worker/activity-environment checks with the time-skipping server: 59 passed, 10 skipped.
• poe lint passes (Ruff formatting/imports, Pyright, mypy, basedpyright and docstyle).

Python source was tested with Python 3.13.16 and the published 1.33.0 native bridge; no Rust code changes. CI should verify the current bridge build.

Prepared with OpenAI Codex.

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