create: implement retries for individual fs files (backport of #7351)

[ThomasWaldmann]

Backport of #7351 to 1.4-maint.

Retry reading a backup source file when it raises a BackupError / BackupOSError:

• retry the same file after a sleep (1ms, growing exponentially up to 10s, 10 tries)
• BackupOSError: if retries don't help, skip the file, log it with E status
• BackupError (changed while reading): last try backs it up, logged with C status

Works for borg create's builtin fs recursion, --paths-from-command and --paths-from-stdin.

1.4-specific care: checkpoint part files

1.4 has checkpointing that may write .borg_part_N files. A naive retry would re-read the file from the start and write a second set of part files, so concatenating all part files would no longer reproduce the complete file. Therefore a file is not retried once part files have been written for it (ChunksProcessor.last_part_number tracks this) — it is kept as-is (C) or skipped (E) instead.

Permission errors

EPERM/EACCES are not retried (they won't get better) — avoids ~15s of pointless backoff on every unreadable file. This is the post-#7351 follow-up behavior; included here to avoid a regression vs. current 1.4.

Tests

Adds a ChunkerFailing test chunker (--chunker-params=fail,block_size,map) to drive the error handling, and:

• test_create_erroneous_file (from create: implement retries for individual fs files #7351) — file is read on a later retry
• test_create_no_permission_file — permission error is not retried, file skipped
• test_create_erroneous_file_with_part_files — new, 1.4-specific — a file with part files is not retried (no master equivalent, since master 2.0 has no part files)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.33%. Comparing base (e4e2229) to head (396b8cd).
⚠️ Report is 3 commits behind head on 1.4-maint.

Files with missing lines	Patch %	Lines
src/borg/archiver.py	65.38%	13 Missing and 5 partials ⚠️
src/borg/archive.py	83.72%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #9812      +/-   ##
=============================================
+ Coverage      82.03%   82.33%   +0.29%     
=============================================
  Files             38       38              
  Lines          11396    11442      +46     
  Branches        1797     1804       +7     
=============================================
+ Hits            9349     9421      +72     
+ Misses          1462     1434      -28     
- Partials         585      587       +2     

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