Fix memory leaks and race conditions when using arun_many() with managed CDP browsers. Each crawl now gets proper page isolation with automatic cleanup while maintaining shared browser context.
Key fixes:
- Close non-session pages after crawling to prevent tab accumulation
- Add thread-safe page creation with locks to avoid concurrent access
- Improve page lifecycle management for managed vs non-managed browsers
- Keep session pages alive for authentication persistence
- Prevent TOCTOU (time-of-check-time-of-use) race conditions
This ensures stable parallel crawling without memory growth or browser instability.
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