perf(file-system): Fix quadratic file rescans in the FS request queue

[vdusek]

Problem

The file system request queue client was O(n²):

• Every cache refresh globbed and parsed every request file ever written (handled files are never deleted from disk).
• forefront=True invalidated the whole cache, and RequestManagerTandem adds every loader request with forefront=True, which triggered a full rescan per processed request.
• The blocking json.load ran on the event loop.

Processing a 10k RequestList through the tandem meant roughly 50M file parses.

Changes

• _refresh_cache now derives pending requests from the recoverable state's key→sequence maps and reads only those files, instead of globbing and parsing the whole directory.
• New forefront requests go straight to the front of the in-memory cache. The cache is invalidated only when an existing request is repositioned, or when it's at capacity.
• Handled requests are pruned from the pending state maps (they stay in handled_requests for deduplication), so cache refreshes, is_empty, and the persisted state blob scale with the backlog rather than with every request ever processed.
• _parse_request_file reads the file in a single asyncio.to_thread call instead of running json.load on the event loop.

Results

Processing 200 requests through the tandem pattern went from 20,100 file parses (2.8 s) to 1 (0.3 s), and scaling is now linear (2,000 requests in about 2.6 s). Ordering semantics (forefront LIFO, regular FIFO), deduplication, and crash-recovery behavior are preserved and covered by new regression tests.

Logging changes

The file system request queue no longer emits the Request <unique_key> not found in state, skipping. warning. The cache refresh no longer scans the queue directory, so orphaned request files (e.g. written by another process) are silently ignored.

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.44%. Comparing base (7780e78) to head (be52914).
⚠️ Report is 46 commits behind head on master.

Files with missing lines	Patch %	Lines
...rage_clients/_file_system/_request_queue_client.py	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2011      +/-   ##
==========================================
+ Coverage   93.35%   93.44%   +0.09%     
==========================================
  Files         179      179              
  Lines       12482    12467      -15     
==========================================
- Hits        11652    11650       -2     
+ Misses        830      817      -13     

Flag	Coverage Δ
unit	93.44% <95.83%> (+0.09%)	⬆️

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