gh-108668: Preserve exception state during delegated throw

[joaogabriel15]

Fixes #108668.

When throw() is forwarded through yield from or await, the delegating generator is linked into the frame stack but its handled-exception state is not linked into tstate->exc_info. The delegated code can consequently inherit the caller's exception instead of the delegating generator's exception.

Link and restore gi_exc_state around both native generator/coroutine delegation and calls to a custom iterator's throw method.

Regression tests cover native generators, a custom iterator wrapper, nested await, and restoration of the caller's handled exception. All three regression scenarios fail on the original Windows debug build and pass with the change.

Validation: Windows x64 debug build; test_yield_from, test_generators, test_coroutines, test_asyncgen, test_genexps, test_exceptions, test_contextlib, and test_asyncio. Result: SUCCESS, 3,163 tests run, 92 skipped; 41 test files passed and the Unix-only file was skipped.

• Issue: exc_info can get lost when throwing into an await or yield from #108668

[python-cla-bot]

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