fix: allow opting out of anti-bot blocking

[nightcityblade]

Summary

Fixes #2058.

Adds a CrawlerRunConfig.check_blocked option, defaulting to True, so callers can opt out of anti-bot classification when they want to apply their own content-quality policy. When disabled, detector matches no longer trigger proxy retries, fallback fetches, or a failed CrawlResult.

List of files changed and why

• crawl4ai/async_configs.py - defines and serializes the safe check_blocked configuration option.
• crawl4ai/async_webcrawler.py - skips anti-bot classification when the option is disabled.
• tests/proxy/test_antibot_opt_out.py - verifies the detector is not called and a small page remains successful.

How Has This Been Tested?

• .venv/bin/pytest -q tests/proxy/test_antibot_opt_out.py — 1 passed.
• .venv/bin/python tests/proxy/test_antibot_detector.py — 47 passed.
• .venv/bin/ruff check --ignore F401,F405 crawl4ai/async_configs.py crawl4ai/async_webcrawler.py tests/proxy/test_antibot_opt_out.py — passed (F401/F405 are pre-existing in the touched modules).
• .venv/bin/black --check --target-version py312 tests/proxy/test_antibot_opt_out.py — passed.

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