Fix duplicate_iterators for async spies

[lprnmns]

Problem

mocker.spy supports async functions and documents spy_return_iter for iterator returns when duplicate_iterators=True. The synchronous spy wrapper honors that option, but the async wrapper returned the original iterator without creating the duplicate, leaving spy_return_iter as None.

Fix

Apply the existing itertools.tee handling in the async spy wrapper when duplicate_iterators=True and the awaited result is an iterator.

The unreleased changelog now records the behavior correction.

Tests

• ./.venv/bin/python -m pytest tests/test_pytest_mock.py::test_async_spy_return_iter_duplicates_iterator_when_enabled -q - failed before the fix as expected; passed after the fix
• ./.venv/bin/tox run -e py312 - passed; 92 tests
• ./.venv/bin/tox run -e norewrite - passed; 88 passed, 4 expected skips
• ./.venv/bin/pre-commit run --all-files - passed
• ./.venv/bin/python -m compileall -q src tests/test_pytest_mock.py - passed
• git diff HEAD^ HEAD --check - passed

Compatibility

The change affects only the existing opt-in duplicate_iterators=True path for async functions. Non-iterable async results and duplicate_iterators=False retain their prior behavior.

Related issue

Independent reproduction; no issue is claimed. Open issue #519 concerns a separate Python-version-specific unittest.mock autospec behavior and was not reproduced on the available Python 3.12 runtime.