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]

All commit authors signed the Contributor License Agreement.

[picnixz]

Can you add a test where you use an async mock with a sync callable object? i.e. AsyncMock + SyncCallable.

[cananoo]

Added a regression test using a synchronous callable object as AsyncMock.side_effect in commit 7e15b35.

[picnixz]

Suggested change

	Await async callable objects used as the ``side_effect`` of
	:class:`unittest.mock.AsyncMock`.
	Correctly handle async callable objects used as the ``side_effect``
	of :class:`unittest.mock.AsyncMock`.

[cananoo]

Applied the suggested NEWS wording in commit 7e15b35.