Enable cache_only mode for the BF-Tree cache

[hildebrandmw]

This was left as a TODO pending a backend fix to bf-tree. That change has since been made, but this was never updated. Without cache_only, a memory-fs will be used which prevents BF-Tree from actually working as a fixed-capacity cache.

[Copilot]

Pull request overview

Enables BF-Tree’s cache_only mode so the caching layer behaves as a true fixed-capacity cache (rather than falling back to an in-memory backing store that undermines eviction).

Changes:

• Switch BF-Tree config to cache_only(true) in the cache constructor.
• Update the cache metrics test expectations for initial utilization.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.45%. Comparing base (f206264) to head (038ad2b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #810   +/-   ##
=======================================
  Coverage   89.45%   89.45%           
=======================================
  Files         432      432           
  Lines       79452    79452           
=======================================
  Hits        71075    71075           
  Misses       8377     8377           

Flag	Coverage Δ
miri	89.45% <ø> (ø)
unittests	89.30% <ø> (ø)

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