Fully support async tests and fixtures

[MatthewMckee4]

Summary

• Add asyncio.run() support so async def test_*() functions are properly awaited instead of silently passing
• Handle async fixtures (non-generator) by wrapping their return coroutine with asyncio.run()
• Handle async generator fixtures by using __anext__() + asyncio.run() for both setup and teardown

Closes #484

Test plan

• test_async_function — basic async test that passes
• test_async_function_with_assertion_error — async test that fails with assertion
• test_async_fixture — async non-generator fixture with async test
• test_async_generator_fixture — async generator fixture with setup/teardown verification
• test_async_test_with_sync_fixture — async test consuming a sync fixture
• test_sync_test_with_async_fixture — sync test consuming an async fixture
• Full test suite (637 tests pass)
• uvx prek run -a passes

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

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_{Comparing async-test-support (56fd2c1) with main (631b238)}