[MOD-17675] Refactor addVector to support reuse flows

[nonirosenfeldredis]

Summary

• Split TieredHNSWIndex::addVector into addVectorInPlace and addVectorToFlat (private helpers), leaving addVector as the dispatcher.
• Move the single-value branch of updateVectors into updateVectorSingleValue, and flatten the multi-value reuse branches.
• Behavior fix: a single-value updateVectors(label, nullptr, 0) now removes the label (as before the refactor) instead of inserting from a null blob.

Test plan

• New test updateVectorsSingleValueZeroVectorsRemovesLabel (async + in-place); verified it crashes without the n == 0 guard.
• Tiered HNSW suite (299 tests) passes on release/macOS, seeds 1-5.
• CI (libstdc++), debug/ASAN builds

🤖 Generated with Claude Code

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Note

Medium Risk
Touches core tiered HNSW insert/update/delete and async job submission paths; the n==0 fix is localized but insertion refactor affects all add/update entry points.

Overview
Refactors tiered HNSW vector insertion and update so reuse / id-reservation flows are easier to follow, without changing the overall tiering model.

addVector is now a thin dispatcher: write-in-place work lives in addVectorInPlace, and flat-buffer enqueue + insert-job creation lives in addVectorToFlat. updateVectors on single-value indexes delegates to new updateVectorSingleValue, which handles n > 1, n == 0, and the single-vector overwrite via addVector. Multi-value reuse paths are grouped under one reuseIdOnUpdate && !sqAccumulationState guard (in-place first, then async when n > 0).

Behavior fix: updateVectors(label, nullptr, 0) on a single-value index removes the label instead of treating a null blob as an overwrite. A unit test covers async (flat) and in-place (HNSW) cases.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.57%. Comparing base (0348684) to head (152c478).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1079      +/-   ##
==========================================
- Coverage   97.58%   97.57%   -0.01%     
==========================================
  Files         142      142              
  Lines        9444     9455      +11     
==========================================
+ Hits         9216     9226      +10     
- Misses        228      229       +1     

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