Fix suppression lifetime for coroutine functions

[adenzhou1350]

Changes

@suppress_type_checks currently restores checking as soon as an async function returns its coroutine object, before its body runs. For example, a decorated async def that calls check_type(1, str) after an await still raises TypeCheckError.

Use an async wrapper for coroutine functions and reuse the existing suppression context manager around the await. Suppression starts on execution, survives suspension, and unwinds on completion, exception or cancellation. Synchronous functions and the existing globally shared suppression semantics are unchanged; this does not introduce task-local isolation or generator support.

Validation

• Five new regression nodes fail on unchanged production code and pass with the fix. They cover direct/instrumented checking, nesting, exceptions and cancellation, plus coroutine identity, argument/return forwarding, creation without activation and restoration.
• Windows CPython 3.12 full suite: 540 passed, 9 skipped, 9 xfailed (warnings treated as errors by project configuration).
• Pinned Ruff 0.15.20 lint/format and git diff --check: pass.
• A clean, non-incremental full-source mypy 2.1.0 run reports the same pre-existing missing typeshed.stdlib.types import at _decorators.py:22 on baseline and candidate. No additional error is reported. Other Python versions and the Sphinx build have not been run locally.

Checklist

• Added failing-before regression tests
• Updated the user guide and API documentation
• Added a changelog entry

[coveralls]

coverage: 94.876% (+0.02%) from 94.853% — adenzhou1350:codex/fix-async-typecheck-suppression into agronholm:master