fix: don't count file cache buffer pool size if disabled

[MrCroxx]

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What's changed and what's your intention?

Don't count file cache buffer pool size if disabled.

close #12943

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.

[hzxa21]

LGTM

[codecov]

Codecov Report

Merging #12941 (0de820f) into main (01ed55b) will increase coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main   #12941      +/-   ##
==========================================
+ Coverage   69.17%   69.19%   +0.01%     
==========================================
  Files        1493     1493              
  Lines      246499   246501       +2     
==========================================
+ Hits       170521   170560      +39     
+ Misses      75978    75941      -37     

Flag	Coverage Δ
rust	69.19% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/compute/src/memory_management/mod.rs	83.43% <100.00%> (+0.19%)	⬆️

... and 8 files with indirect coverage changes

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