Fix flaky AsyncWrapper reset_state test on Python 3.14

[bvolpato]

Fixes #39456.

Summary

The deadlock regression test currently starts its 10-second timeout at process creation. That timeout includes child startup and imports, even though failure is reported as a reset_state() deadlock. Python 3.14 uses forkserver by default on POSIX, making this distinction more relevant.

This change adds a readiness signal immediately before reset_state(). Parent uses a separate startup timeout, then applies existing 10-second limit only to teardown. This preserves real deadlock detection and gives distinct failures for startup and teardown. Production code is unchanged.

Validation

• Python 3.14 cloud tox target: 2 passed
• Python 3.13 tox target: 2 passed
• Python 3.14 cloud stress: 100/100 passes for each asyncio and thread-pool variant
• Negative control: injected 11-second stall after readiness signal; test failed at original 10-second deadlock assertion
• YAPF 0.43.0 and Ruff 0.15.7 passed on changed file

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[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[github-actions]

Assigning reviewers:

R: @jrmccluskey for label python.

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[tvalentyn]

Thanks, @bvolpato!

filed #39526 for another flake in Python 3.10