remove the drop_corrupt_tail mechanism, refs #10369

[mr-raj12]

Description

Follow-up to #10369, review point 3. Every check --repair rebuild of the chunk index now has an object validator, so drop_corrupt_tail was only reachable from tests.

Depends on #10369. This branch contains its commit; I will rebase onto master after #10369 is merged.

Changes

• PackReader.iter_headers(): remove the drop_corrupt_tail parameter. Without a validator, a corrupt object header raises IntegrityError.
• build_chunkindex_from_repo(): remove the drop_corrupt_tail parameter.
• Repository: remove chunkindex_drop_corrupt_tail.
• ArchiveChecker.check(): stop passing drop_corrupt_tail=repair and setting repository.chunkindex_drop_corrupt_tail.

No behaviour change: a walk without a validator only happens in a check without --repair, and it already raised.

Tests

• Removed:
  • repository_test.py::test_pack_reader_drops_a_corrupt_tail_only_when_asked
  • repository_test.py::test_pack_reader_drops_a_corrupt_tail_without_an_on_drop
  • repository_test.py::test_pack_reader_drop_corrupt_tail_does_not_affect_a_validating_walk
  • cache_test.py::test_build_chunkindex_without_a_validator_drops_the_rest_of_a_damaged_pack
  • cache_test.py::test_build_chunkindex_without_drop_corrupt_tail_raises_on_a_damaged_pack
• check_cmd_test.py::test_check_without_repair_does_not_drop_a_pack_tail renamed to test_check_without_key_aborts_on_a_corrupt_pack_header. It asserts that the rebuild gets no validator and raises CorruptPack.
• Still covered by unchanged tests: test_pack_reader_raises_on_bad_magic (raises without a validator) and test_build_chunkindex_repair_resyncs_after_corrupt_header (CorruptPack without a validator; with one, the rebuild skips the damaged object and indexes the next one).

TZ=UTC pytest --benchmark-skip -n auto: 3072 passed, 1141 skipped.

Note for #10368

#10368 still passes drop_corrupt_tail=repair and sets chunkindex_drop_corrupt_tail in ArchiveChecker.check(). Whichever PR merges second removes those two lines.

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 88.11%. Comparing base (3b905e0) to head (624ccdc).
⚠️ Report is 35 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10378      +/-   ##
==========================================
- Coverage   88.12%   88.11%   -0.02%     
==========================================
  Files         103      103              
  Lines       18928    18843      -85     
  Branches     2923     2911      -12     
==========================================
- Hits        16681    16604      -77     
+ Misses       1556     1554       -2     
+ Partials      691      685       -6     

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