repository: raise PackNotFound when get_many hits a missing pack

[mr-raj12]

get_many() now raises PackNotFound instead of ObjectNotFound when a chunk's index entry points at a pack that's missing from the store. That's a different failure than a single missing object: it usually means the index is stale, or we've lost more than one chunk's worth of data. follow up for review comment on #9836.

Added a test for it: delete a pack but leave its index entry, then check PackNotFound fires, and that raise_missing=False still just yields None.

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.12%. Comparing base (230f6ef) to head (cbc94ae).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/repository.py	77.77%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9864      +/-   ##
==========================================
+ Coverage   85.09%   85.12%   +0.02%     
==========================================
  Files          93       93              
  Lines       15450    15458       +8     
  Branches     2335     2337       +2     
==========================================
+ Hits        13147    13158      +11     
+ Misses       1604     1599       -5     
- Partials      699      701       +2     

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