fix(vector): Make construction pruning deterministic and align tests

[joelamming]

Description

This PR is a small follow-up to @matthewmcneely's matthewmcneely/fix-similarity-based-hnsw-indexing that tightens the construction-time pruning path in tok/hnsw/helper.go:addNeighbors.

• Makes construction pruning explicitly maintain the best-to-worst top‑k neighbour list capped at efConstruction (rather than relying on heap/slice side-effects)
• Avoids duplicate/self edges during construction updates
• Aligns the pruning unit test with the same helper used by production to reduce test/production drift

This is intended to be easy to cherrypick into the PR branch

Why

The directionality fix is the key part (and @matthewmcneely's PR nails that), but the existing pruning loop still had a couple of structure-level issues (heap initialisation/slice truncation semantics) that could make construction behaviour hard to reason about -- especially under similarity metrics

Testing

• go test ./tok/hnsw -count=1
• Synthetic recall harness (25k vectors) shows massively improved recall vs. main (per my comment) and stable miss-rate and expected build/query behaviour vs. fix(vector): Fix similarity-based HNSW search for cosine and dot product metrics #9559

Checklist

• The PR title follows the
  Conventional Commits syntax, leading
  with fix:, feat:, chore:, ci:, etc.
• Code compiles correctly and linting (via trunk) passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable