tests(feat[async_doctest]): Add comprehensive async doctest tests

why: Verify async doctest functionality works correctly
what:
- Use functional pytest style with NamedTuple parametrization
- Add RST and Markdown dual coverage for all test cases
- Test basic top-level await with asyncio.sleep
- Test async functions returning values
- Test mixed sync/async examples in same block
- Test state persistence across examples
- Test async context managers (async with)
- Test async iteration (async for)
- Test async comprehensions
- Test expected exceptions in async code
- Add pytester-based plugin integration tests
- Test conftest fixture integration with async
- Test failure reporting for async doctests

Author: tony