testsuite: add UpgraderFrom12To20 unit tests, refs #9556

[mr-raj12]

Description

Unit tests for UpgraderFrom12To20, refs #9556.

UpgraderFrom12To20 (legacy/upgrade.py) shipped in earlier phases without any test coverage. This PR adds src/borg/testsuite/legacy_upgrade_test.py covering all three public methods.

src/borg/testsuite/legacy_upgrade_test.py -- 20 tests for UpgraderFrom12To20:

upgrade_item (8 tests):

• regular file passes through unchanged
• whitelist strips legacy-only keys (acl, chunks_healthy, hardlink_master)
• user/group set to None are dropped
• symlink source renamed to target
• hardlink master gets hlid assigned and hardlink_master key removed
• hardlink slave resolves hlid and reuses chunks via cache.reuse_chunk
• master and slave share the same hlid
• REQUIRED_ITEM_KEYS always present in output

upgrade_archive_metadata (9 tests):

• cmdline list becomes command_line string
• recreate_cmdline list becomes recreate_command_line string
• time gets +00:00 UTC offset appended
• 4-tuple chunker_params gets buzhash prefix
• 5-tuple chunker_params passes through unchanged
• rechunking override replaces stored chunker_params
• recreate fields (recreate_source_id, recreate_args, recreate_partial_chunks) dropped
• tags always set to []
• missing optional attrs absent from result

upgrade_compressed_chunk (3 tests):

• ZLIB-legacy data detected and ctype promoted to ZLIB.ID
• non-ZLIB chunks have two prefix bytes stripped
• ObfuscateSize big-endian csize header re-parsed correctly; inner payload and padding preserved

Also fixes src/borg/compress.pyi: CompressorBase.ID was typed as bytes but is int at runtime in every subclass.

No production code changes.

Refs #9556

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.99%. Comparing base (828e72b) to head (b7c8417).
⚠️ Report is 6 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9675      +/-   ##
==========================================
+ Coverage   83.82%   83.99%   +0.16%     
==========================================
  Files          93       93              
  Lines       15592    15592              
  Branches     2337     2337              
==========================================
+ Hits        13070    13096      +26     
+ Misses       1786     1771      -15     
+ Partials      736      725      -11     

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[ThomasWaldmann]

Thanks for adding these.

BTW, it's not correct there were no test for the upgrader, there is a quite big and quite practical one inside the transfer_cmd tests, do a real repository upgrade from 1.2 to 2.

[ThomasWaldmann]

LGTM.