Enable Crawl4AI to connect to pre-created CDP browser contexts, which is
essential for cloud browser services that pre-create isolated contexts.
Changes:
- Add browser_context_id and target_id parameters to BrowserConfig
- Update from_kwargs() and to_dict() methods
- Modify BrowserManager.start() to use existing context when provided
- Add _get_page_by_target_id() helper method
- Update get_page() to handle pre-existing targets
- Add test for browser_context_id functionality
This enables cloud services to:
1. Create isolated CDP contexts before Crawl4AI connects
2. Pass context/target IDs to BrowserConfig
3. Have Crawl4AI reuse existing contexts instead of creating new ones
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