fix: send async logger output to stderr

[nightcityblade]

Summary

Fixes #1968

Routes AsyncLogger console output to stderr by default so stdout remains clean for stdio protocols such as MCP, while still allowing callers to pass a custom Rich console.

List of files changed and why

• crawl4ai/async_logger.py - Default logger console output to stderr and add an optional custom console parameter.
• tests/loggers/test_async_logger_stderr.py - Cover stderr default behavior and custom console override.

How Has This Been Tested?

• . .venv/bin/activate && python -m pytest tests/loggers/test_async_logger_stderr.py

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes