fix: wrong num_entities used when mmap variable length data (#30848)

[longjiquan]

#30728
pr: #30848

[mergify]

@longjiquan E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[longjiquan]

/run-cpu-e2e

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.25%. Comparing base (7105e0b) to head (3e0e915).
Report is 5 commits behind head on 2.3.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.3   #31274      +/-   ##
==========================================
+ Coverage   82.24%   82.25%   +0.01%     
==========================================
  Files         844      834      -10     
  Lines      122234   122151      -83     
==========================================
- Hits       100526   100480      -46     
+ Misses      18480    18437      -43     
- Partials     3228     3234       +6     

Files	Coverage Δ
internal/core/src/mmap/Column.h	95.26% <100.00%> (-2.24%)	⬇️

... and 167 files with indirect coverage changes

[czs007]

/approve
/lgtm

[sre-ci-robot]

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