remote: fix StoreObjectNotFound exception lost over RPC, fixes #9380

[mr-raj12]

Summary

• RemoteRepository RPC layer serializes exceptions using e.__class__.__name__, but both borgstore.store.ObjectNotFound and Repository.ObjectNotFound share __name__ == "ObjectNotFound"
• Client-side handle_error always reconstructs Repository.ObjectNotFound, even when the server actually raised StoreObjectNotFound from a store_load()/store_delete() call
• This was already acknowledged with TODO comments at 3 sites in cache.py: # TODO: ^ seem like RemoteRepository raises Repository.ONF instead of StoreONF
• Fix: distinguish StoreObjectNotFound during server-side serialization and reconstruct it correctly on the client side

Fixes #9380

Changes

File	Change
src/borg/remote.py	Import StoreObjectNotFound; serialize it as "StoreObjectNotFound" (not ambiguous "ObjectNotFound") on the server side; add client-side handle_error branch to reconstruct it; add inject_exception test hook
src/borg/cache.py	Simplify 3 except clauses from (Repository.ObjectNotFound, StoreObjectNotFound) to StoreObjectNotFound; remove TODO comments
src/borg/testsuite/repository_test.py	Add StoreObjectNotFound round-trip test in test_remote_rpc_exception_transport

Backward compatibility note

This changes the RPC wire format for StoreObjectNotFound:

• Old server → new client: old server sends "ObjectNotFound", new client raises Repository.ObjectNotFound - cache.py callers would miss it. In practice, store_* methods are borg2-only and borg2 is in beta, so mixed-version deployments are not a supported scenario.
• New server → old client: old client falls through to RPCError fallback.

Checklist

• PR is against master
• New code has tests
• Tests pass
• Commit message references related issue
• Code formatted with black

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.47%. Comparing base (cace06f) to head (1935d75).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/cache.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9381      +/-   ##
==========================================
+ Coverage   76.44%   76.47%   +0.03%     
==========================================
  Files          85       85              
  Lines       14799    14801       +2     
  Branches     2212     2213       +1     
==========================================
+ Hits        11313    11319       +6     
+ Misses       2808     2804       -4     
  Partials      678      678              

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[ThomasWaldmann]

great, some TODOs less!