fix(browsers): do not close a browser as inactive while a page opening is in flight

[vdusek]

The windows-latest / 3.10 unit-test job failed with TargetClosedError: BrowserContext.new_page: Target page, context or browser has been closed in test_new_page_with_each_plugin. It is not a test flake: the BrowserPool inactive-browser reapers judge a browser only by its finished pages (idle_time runs from controller construction and pages is populated only once new_page returns), so on a slow runner they gracefully close a freshly launched browser whose first new_page() is still creating its context. Any user on a loaded machine can hit the same error (related: #1660).

The fix exposes the already-tracked in-flight openings as BrowserController.is_opening_pages and guards both reapers with it, so such a browser is neither marked inactive nor closed. A new regression test reproduces the race deterministically by gating the first context.new_page() call and running one reaper cycle while the opening is pending — it fails with the exact CI error before the fix and passes after.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.61%. Comparing base (091410f) to head (c5def7b).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/crawlee/browsers/_browser_controller.py	66.66%	1 Missing ⚠️
src/crawlee/browsers/_browser_pool.py	50.00%	1 Missing ⚠️
.../crawlee/browsers/_stagehand_browser_controller.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2156      +/-   ##
==========================================
+ Coverage   93.51%   93.61%   +0.10%     
==========================================
  Files         181      181              
  Lines       12653    12664      +11     
==========================================
+ Hits        11832    11855      +23     
+ Misses        821      809      -12     

Flag	Coverage Δ
unit	93.61% <76.92%> (+0.10%)	⬆️

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[Mantisus]

LGTM