When using create_isolated_context=False with concurrent crawls, multiple
tasks would reuse the same page (pages[0]) causing navigation race
conditions and "Page.content: Unable to retrieve content because the
page is navigating" errors.
Changes:
- Add _pages_in_use set to track pages currently being used by crawls
- Rewrite get_page() to only reuse pages that are not in use
- Create new pages when all existing pages are busy
- Add release_page() method to release pages after crawl completes
- Update cleanup paths to release pages before closing
This maintains context sharing (cookies, localStorage) while ensuring
each concurrent crawl gets its own isolated page for navigation.
Includes integration tests verifying:
- Single and sequential crawls still work
- Concurrent crawls don't cause race conditions
- High concurrency (10 simultaneous crawls) works
- Page tracking state remains consistent
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