fix(aio): drain blocked senders when closing channels

[Kuang-xianxin]

Closing a full bounded Chan wakes blocked senders but leaves their completed futures in _puts. A second close(), either immediately or after the senders finish, calls set_exception() on those futures and raises InvalidStateError during shutdown.

Drain the sender waiters when closing and only signal unfinished futures. Regression tests exercise both close timings through the public channel API, verify that all blocked senders receive ChanClosed, and that the buffered item remains readable.

Validation:

• Both regressions fail on unchanged 34a4e8f; the 12 existing tests in test_aio.py pass there.
• All 19 tests in tests/test_aio.py and tests/test_aio_itertools.py pass on Python 3.13.14.
• Repository-wide Ruff checks and formatting (1,041 files), scoped Linux-target mypy for the channel module, and git diff --check pass.
• The configured all-plugin type check stops at the local environment's missing livekit.plugins.anam py.typed marker, identically on the unchanged base. The full SDK/provider integration suite was not run locally.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.