enhance: improve reduce performance on very large topk

[xiaofan-luan]

fix #32870
reduce cpu usage for reduce on large k

[mergify]

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.08%. Comparing base (8a9a421) to head (0005af4).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #32871      +/-   ##
==========================================
+ Coverage   82.07%   82.08%   +0.01%     
==========================================
  Files        1007     1007              
  Lines      127647   127647              
==========================================
+ Hits       104767   104780      +13     
+ Misses      18903    18895       -8     
+ Partials     3977     3972       -5     

Files	Coverage Δ
internal/proxy/search_reduce_util.go	84.00% <100.00%> (ø)

... and 35 files with indirect coverage changes

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xiaofan-luan, Yherealtita

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[mergify]

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

[mergify]

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

[xiaofan-luan]

/run-cpu-e2e

[czs007]

/lgtm