check: handle Ctrl-C at safe boundaries (#7893)

[mr-raj12]

borg check had no signal handling. The first Ctrl-C did nothing, because nothing polled the soft-interrupt flag, and the second one raised KeyboardInterrupt from wherever execution happened to be. Under --repair that could land mid-rebuild, or before finish() ran, which leaves a stale chunk index behind (#9850).

Now the pack scan, verify_data, rebuild_archives_directory and rebuild_archives check sig_int and stop at a safe boundary: chunk boundaries for the read-only scans, between whole archives for the repair rebuild. The repository check saves its PackTracker so a later run picks up where it left off. finish() runs before we raise either way, so the index and manifest stay consistent. This follows what compact already does (#9890).

Includes tests for both the read-only and --repair interrupt paths.

Fixes #7893.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.71%. Comparing base (a50eb6f) to head (a62911e).
⚠️ Report is 8 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archive.py	83.33%	1 Missing and 1 partial ⚠️
src/borg/archiver/check_cmd.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9966      +/-   ##
==========================================
+ Coverage   85.70%   85.71%   +0.01%     
==========================================
  Files          95       95              
  Lines       16815    16860      +45     
  Branches     2577     2588      +11     
==========================================
+ Hits        14411    14452      +41     
- Misses       1668     1671       +3     
- Partials      736      737       +1     

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