test(partitions): reproduce purge offset directory sync failure

[diegomrsantos]

Purge can return success and record its applied generation after syncing a consumer offset directory fails. A later power loss can restore the deleted offset files while the completion marker survives, leaving old consumer progress with no purge retry pending.

Refs #4128.

This draft adds regression tests against master at e0027506f2a0379ee3bb5508427ff570e3a9a435. It contains no production fix.

The tests call the real purge() method and inject a sync failure for each offset directory separately. The fixture persists progress for an individual consumer and a consumer group, records generation 4, then requests generation 5. It checks that the fault was reached and the offset files were deleted, then reloads the applied generation into a fresh partition.

Both failure cases expect purge to return an error and leave generation 4 in memory and on disk. Instead, purge returns success and both generations advance to 5. A third test allows both directory syncs to succeed and confirms generation 5 is recorded.

The two failure tests intentionally remain red until the bug is fixed. This PR reloads only the generation marker. It does not simulate power loss, restore deleted offset files, reload consumer progress, check Next, or test retries with fresh writes. The fixture writes durable offset files but does not run both consumer offset durability policies. The simulator coverage proposed in the issue discussion would cover those remaining cases.

Run the focused reproduction with

cargo test -p partitions purge_offset_directory_sync_tests -- --nocapture

The original macOS run produced one passing control and two expected failures. CI also reproduces the group directory failure. Formatting, dependency ordering, Clippy, and TOML checks pass in CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.47%. Comparing base (ad8951f) to head (33c07b3).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4130      +/-   ##
============================================
- Coverage     87.47%   87.47%   -0.01%     
  Complexity     1575     1575              
============================================
  Files          1280     1280              
  Lines        223241   223230      -11     
  Branches     186604   186593      -11     
============================================
- Hits         195290   195273      -17     
- Misses        23241    23250       +9     
+ Partials       4710     4707       -3     

Components	Coverage Δ
Rust Core	88.54% <ø> (-0.01%)	⬇️
Java SDK	68.68% <ø> (ø)
C# SDK	77.41% <ø> (-0.05%)	⬇️
Python SDK	90.97% <ø> (ø)
PHP SDK	85.67% <ø> (ø)
Node SDK	96.43% <ø> (-0.02%)	⬇️
Go SDK	70.11% <ø> (-0.02%)	⬇️

Files with missing lines	Coverage Δ
core/partitions/src/iggy_partition.rs	92.83% <ø> (+<0.01%)	⬆️

... and 6 files with indirect coverage changes

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