Fix miss index file while slice meta contains raw data only

[yah01]

/kind bug
related #26178 #25780 #26131 #25905

[yah01]

Also the slice meta is really a bad and stupid design, there are even 3 cases for such simple logic:

• no slice meta if index && raw data files are both small
• slice meta contains raw data only if the index file is small
• slice meta contains both files if they are both large

Actually I guess the 4-th case exists, but it's hidden by the property: raw data is always larger than the index itself.

Proposal:
The slice meta should be always generated, even there is only 1 small file.
Simple and easy to handle

[mergify]

@yah01 E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[congqixia]

/lgtm

[codecov]

Codecov Report

Merging #26190 (624dbcb) into master (28cea5e) will decrease coverage by 0.02%.
Report is 15 commits behind head on master.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##           master   #26190      +/-   ##
==========================================
- Coverage   82.18%   82.16%   -0.02%     
==========================================
  Files         807      812       +5     
  Lines      109620   111077    +1457     
==========================================
+ Hits        90089    91269    +1180     
- Misses      16454    16685     +231     
- Partials     3077     3123      +46     

Files Changed	Coverage Δ
internal/core/src/index/VectorMemIndex.cpp	66.99% <78.57%> (-0.02%)	⬇️

... and 32 files with indirect coverage changes

[mergify]

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[mergify]

@yah01 E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[mergify]

@yah01 E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[jiaoew1991]

/lgtm
/approve

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