check: treat v2 archive metadata without item_ptrs as broken, refs #8649

[ThomasWaldmann]

Authenticated v2 archive metadata that lacks item_ptrs crashed borg check with AttributeError: attribute item_ptrs not found, because the archive loop only verified archive.version before reading archive.item_ptrs.

check now treats v2 archive metadata without item_ptrs, or with the v1 items key as well, like other broken archive metadata (missing or integrity error):

• without --repair: logs Archive metadata block <id> is invalid: needs item_ptrs and no items plus Would delete broken archive ..., flags an error and continues with the next archive.
• with --repair: deletes the archive, consistent with the existing handling of unusable archive metadata.

Rejecting the "both keys" case is consistent with the "exactly one of item_ptrs, items" check that #10522 adds to Archive._load_meta / Archives._parse_archive_meta; otherwise check --repair would rebuild such an archive with the stale items key kept.

Test: test_archive_metadata_invalid_item_ptrs in check_cmd_test.py (variants: no item_ptrs, both keys) fails on master and passes with the fix.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.34%. Comparing base (0638531) to head (5194e28).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10523      +/-   ##
==========================================
+ Coverage   89.32%   89.34%   +0.01%     
==========================================
  Files         104      104              
  Lines       20079    20088       +9     
  Branches     3152     3154       +2     
==========================================
+ Hits        17936    17947      +11     
+ Misses       1484     1481       -3     
- Partials      659      660       +1     

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