fix: correct HNSW frontier comparisons in hybrid cache

[cryo-zd]

What type of PR is this?

fix: correct HNSW frontier comparisons in hybrid cache

What this PR does / why we need it:
  This PR fixes the sign bug in both func (h *HybridCache) searchLayerHybrid and func (h *HybridCache) searchLayerHybridWithEarlyStop: the code compared currentDist against -results.data[0].dist even though distances were already stored as negatives, so saturated frontiers kept enqueuing worse neighbors instead of pruning them.
  ( the stopping condition (currentDist > -results.data[0].dist) almost never fires, and the “is this better than the frontier?” check (dist < -results.data[0].dist) is nearly always true.)
  This prevents needless graph traversal, keeps lookup latency under control, and avoids returning responses routed through poor matches.

semantic-router/src/semantic-router/pkg/cache/hybrid_cache.go

Lines 927 to 951 in f8f042d

	// searchLayerHybrid searches for nearest neighbors at a specific layer
	func (h *HybridCache) searchLayerHybrid(query []float32, ef int, layer int, entryPoints []int) []int {
	// Reuse buffers from pool to reduce allocations
	buf := getSearchBuffers()
	defer putSearchBuffers(buf)
	visited := buf.visited
	candidates := buf.candidates
	results := buf.results
	for _, ep := range entryPoints {
	if ep < 0 || ep >= len(h.embeddings) {
	continue
	}
	dist := -dotProduct(query, h.embeddings[ep])
	candidates.push(ep, dist)
	results.push(ep, dist)
	visited[ep] = true
	}
	for len(candidates.data) > 0 {
	currentIdx, currentDist := candidates.pop()
	if len(results.data) > 0 && currentDist > -results.data[0].dist {
	break
	}

Which issue(s) this PR fixes:

Fixes #

Release Notes: Yes/No

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[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 src

Owners: @rootfs, @Xunzhuo, @wangchen615
Files changed:

• src/semantic-router/pkg/cache/cache_test.go
• src/semantic-router/pkg/cache/hybrid_cache.go

---

🎉 Thanks for your contributions!

This comment was automatically generated based on the OWNER files in the repository.

[Copilot]

Pull Request Overview

This PR fixes a critical bug in the HNSW (Hierarchical Navigable Small World) search algorithm for the hybrid cache. The issue was with incorrect distance comparisons that caused the search to accept weaker candidates and fail to properly prune branches, leading to inefficient searches and potentially incorrect results.

Key changes:

• Removed redundant negations in distance comparisons (lines 949, 967, 1078, 1101)
• Added clarifying comments explaining the negative dot product convention (lines 941, 1065)
• Added comprehensive test coverage to verify proper pruning behavior

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/semantic-router/pkg/cache/hybrid_cache.go	Fixed distance comparison logic by removing double negations in searchLayerHybrid and searchLayerHybridWithEarlyStop functions; added explanatory comments
src/semantic-router/pkg/cache/cache_test.go	Added regression test to ensure weak branches are properly pruned during hybrid layer search

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[rootfs]

@cryo-zd thank you for testing this!