refactor: Remove inner Arc from FileCacheEntry

[nameexhaustion]

We already always wrap it in an Arc on the outside (Arc<FileCacheEntry>)

[codspeed-hq]

CodSpeed Performance Report

Merging #16870 will improve performances by 17.28%

_{Comparing nameexhaustion:file-cache-arc (db804dc) with main (84e2ea4)}

Summary

⚡ 1 improvements
✅ 36 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	nameexhaustion:file-cache-arc	Change
⚡	test_tpch_q14	2.5 ms	2.1 ms	+17.28%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.41%. Comparing base (0b0af39) to head (db804dc).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16870      +/-   ##
==========================================
+ Coverage   81.33%   81.41%   +0.07%     
==========================================
  Files        1425     1425              
  Lines      187867   187926      +59     
  Branches     2702     2705       +3     
==========================================
+ Hits       152797   152994     +197     
+ Misses      34574    34436     -138     
  Partials      496      496              

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