Repository.close(): don't mask the original exception when unwinding with buffered chunks

[ThomasWaldmann]

Problem

Repository.close() asserts that the PackWriter has no unflushed chunks. When a borg command aborts with chunks still buffered (e.g. an exception mid-borg create between pack flushes), the with repository: unwind calls close() and this assertion raises AssertionError, masking the original exception. Additionally, the buffered chunks left F_PENDING entries in the chunk index, which the close()-time write_chunkindex_to_repo would trip over (it asserts no entry is still pending).

Reproduce: open a Repository, put() one small chunk (below the pack size limit), raise inside the with-block — the AssertionError replaces the real error.

Fix

• New PackWriter._drop_buffered(): empties the piece buffer and deletes the still-pending index entries for the buffered chunks. flush()'s store-failure path now reuses it instead of duplicating the same drop logic inline.
• Repository.__exit__ calls it when unwinding an exception, before close(). The never-stored chunks die with the aborted operation: the original exception propagates unmasked, and no F_PENDING entries reach the persisted chunk index.
• The assertion in close() is unchanged: on a clean close it still catches a genuinely forgotten flush(). __exit__ is the only place that knows whether we are unwinding, so the drop decision lives there.

Tests

• test_exception_unwind_drops_buffered_chunks: the repro above; asserts the original ValueError (not AssertionError) propagates, and after reopening the buffered chunk is neither in the chunk index nor readable.
• test_close_with_unflushed_chunks_asserts: documents that a clean exit with buffered chunks still trips the assertion.

All repository and cache tests pass; ruff clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.10%. Comparing base (2a5cb4c) to head (3d0aff0).
⚠️ 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	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10013   +/-   ##
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  Coverage   86.10%   86.10%           
=======================================
  Files          96       96           
  Lines       17326    17332    +6     
  Branches     2649     2650    +1     
=======================================
+ Hits        14918    14924    +6     
  Misses       1667     1667           
  Partials      741      741           

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