fix: allow Actor.reboot() to be retried after a failed or cancelled attempt

[vdusek]

Description

• Actor.reboot() set the internal _is_rebooting flag before doing any work and never reset it on failure. When the reboot API call raised (or the precondition check failed), the flag stayed True and every subsequent reboot() call was silently skipped for the rest of the run, making retries impossible.
• The run-id precondition check now runs before the flag is set, and the whole reboot sequence is wrapped so that any failure or cancellation resets the flag before re-raising. After a successful reboot the flag intentionally stays set, so duplicate reboot calls remain suppressed while the platform stops the container.
• custom_after_sleep=timedelta(0) is now honored instead of being replaced by the default.
• Adds regression tests covering retry after a failed attempt and after a cancelled attempt.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.89%. Comparing base (2cc5602) to head (d69d0ac).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/apify/_actor.py	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #968      +/-   ##
==========================================
- Coverage   89.91%   89.89%   -0.02%     
==========================================
  Files          49       49              
  Lines        3085     3089       +4     
==========================================
+ Hits         2774     2777       +3     
- Misses        311      312       +1     

Flag	Coverage Δ
e2e	35.93% <0.00%> (-0.05%)	⬇️
integration	56.87% <0.00%> (-0.11%)	⬇️
unit	78.73% <95.83%> (+0.02%)	⬆️

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