testsuite: add Archives and LegacyArchives unit tests, refs #9556

[mr-raj12]

Description

Adds unit tests for Archives and LegacyArchives, refs #9556.

Phase 3 (#9644) merged without test coverage for the new Archives-only code paths and without any tests for LegacyArchives. This PR adds both.

src/borg/testsuite/archives_test.py — 70+ tests for Archives (Borg 2.x, manifest.py):

• init, raw-dict round-trip (_set_raw_dict / _get_raw_dict)
• ids, count, names, exists, exists_id, exists_name_and_id, exists_name_and_ts
• create (str ts, datetime ts, duplicate guard, overwrite=True)
• get / get_by_id (happy path, missing → None, raw=True)
• delete_by_id, undelete_by_id, nuke_by_id
• _get_archive_meta (ObjectNotFound path, success path, bad version)
• list() — sort, reverse, first/last pagination, date filters (older/newer/oldest/newest), all match prefixes (name:, user:, host:, tags:, aid:, ambiguous aid:)
• get_one (exact match, zero matches, multiple matches)
• list_considering (name-conflict guard, full delegation)
• regression: _matching_info_tuples must materialise _info_tuples() via list() before returning — if removed, the raw generator reaches .sort() and raises AttributeError

src/borg/testsuite/legacy_archives_test.py — 50+ tests for LegacyArchives (legacy/archives.py), same coverage plus:

• prepare / finish round-trip
• NotImplementedError stubs (exists_id, exists_name_and_id, exists_name_and_ts, delete_by_id, undelete_by_id, nuke_by_id)
• protocol check: isinstance(la, ArchivesInterface)
• integration smoke: Manifest picks LegacyArchives for a LegacyRepository

No production code changes.

Refs #9556

Checklist

• PR is against master
• New code has tests (this PR is the tests)
• Tests pass
• Commit message references related issue

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.84%. Comparing base (6659bd9) to head (fdc00d9).
⚠️ Report is 16 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9648      +/-   ##
==========================================
+ Coverage   83.08%   83.84%   +0.76%     
==========================================
  Files          90       90              
  Lines       15653    15653              
  Branches     2356     2356              
==========================================
+ Hits        13006    13125     +119     
+ Misses       1887     1788      -99     
+ Partials      760      740      -20     

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