chore: Update pyo3-async-runtimes submodule

Refactored code for upstream PR review:
- Add section comments for better code organization
- Remove debug eprintln! statements
- Add get_handle() as the recommended public API
- Improve documentation for all public functions
- Mark get_runtime() as deprecated with migration guidance

PR: PyO3/pyo3-async-runtimes#71

Author: achimnol