fix: Fix _is_empty_cache invalidation in FileSystemRequestQueueClient

[vdusek]

Summary

• Fix _is_empty_cache race condition in FileSystemRequestQueueClient where fetch_next_request was the only state-mutating method that didn't invalidate the cache, potentially causing stale is_empty() results
• Add cache invalidation to fetch_next_request (conditionally, only when a request is actually found, to avoid hot-path waste)
• Remove redundant early cache invalidation in add_batch_of_requests
• Fix cache/return mismatch in is_empty fast path where _is_empty_cache was set to True but return value could theoretically differ

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.25%. Comparing base (8b53e27) to head (a4c1134).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...rage_clients/_file_system/_request_queue_client.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1840      +/-   ##
==========================================
- Coverage   92.27%   92.25%   -0.02%     
==========================================
  Files         157      157              
  Lines       10909    10909              
==========================================
- Hits        10066    10064       -2     
- Misses        843      845       +2     

Flag	Coverage Δ
unit	92.25% <50.00%> (-0.02%)	⬇️

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