gh-156460: Await async callable side effects in AsyncMock

[cananoo]

AsyncMock did not await callable instances with an asynchronous __call__
method when they were used as side_effect. It returned the coroutine created
by the callable instead, which could result in an unawaited coroutine warning.

This change recognizes an asynchronous __call__ method and awaits the
callable result, while preserving the existing behavior for synchronous
callables and ordinary awaitable return values.

Tests:

• ./python.exe -m test test_unittest.testmock.testasync -j 0
• ./python.exe -m test test_unittest -j 0
• ./python.exe Tools/patchcheck/patchcheck.py

I used OpenAI Codex to assist with investigating and preparing this change,
and reviewed the resulting code, tests, and documentation before submission.

• Issue: async callable not awaited in side_effect for AsyncMock #156460

[python-cla-bot]

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• 2059997196@qq.com