fix: do not add a blank line before async with

[Johnkothapalli]

Summary

• require async def rather than any indented async statement when classifying nested definitions
• preserve the existing nested async def behavior
• add classifier and end-to-end regressions showing that async with does not gain a blank line after a function docstring

Fixes #351.

Validation

• tox -e py314 under Ubuntu/WSL — 241 unit tests passed (1 skipped), 271 integration tests passed (2 skipped), and 40 system tests passed
• pre-commit relevant hooks — merge/TOML/YAML/debug checks, Black, isort, docformatter, Ruff, manifest validation, eradicate, and rstcheck passed
• git diff --check — passed

The full lint workflow on upstream master is already red (run 31345372152) because CHANGELOG.md is changed by end-of-file-fixer, _is_blank_line has an existing D301 finding, and configuration.py has an existing MyPy assignment error. This patch does not change those lines or include their unrelated cleanup.

AI assistance: Codex helped trace the classifier path and prepare the focused tests. The result was verified with the repository's Ubuntu tox gate above.