fix(crawlers): Keep the crawler usable after a failed run

[vdusek]

Follow-up to #2229, split out of #2239.

Crawler state after a failed run

• An exception during setup (e.g. add_requests) left _running set, so every later run() raised RuntimeError. The whole run body is now inside the try/finally.
• After an earlier successful run, a failed run's implicit purge dropped the requests it left pending. A _last_run_failed flag now skips that purge, even with purge_request_queue=True. A failure in the post-crawl steps (statistics, _save_crawler_state()) counts as a failed run.

AutoscaledPool

• run.result.exception() is guarded against a cancelled future. On SIGINT it raised CancelledError in the orchestrator's finally, and the error log was lost.
• The orchestrator's fallback error message now reads Unpropagated exception in worker task orchestrator, so it can be told apart from the same message in run().

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 88.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.73%. Comparing base (45bb0e2) to head (f3c83e5).

Files with missing lines	Patch %	Lines
src/crawlee/crawlers/_basic/_basic_crawler.py	87.50%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2255      +/-   ##
==========================================
- Coverage   93.74%   93.73%   -0.01%     
==========================================
  Files         181      181              
  Lines       12884    12890       +6     
==========================================
+ Hits        12078    12083       +5     
- Misses        806      807       +1     

Flag	Coverage Δ
unit	93.73% <88.00%> (-0.01%)	⬇️

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