test: add dimension mismatch handling tests for semantic cache

[bug-ops]

Summary

Add 6 tests to crates/zeph-memory/src/response_cache.rs to verify that cosine_similarity() and get_semantic() gracefully handle embedding dimension mismatches.

Fixes #2034. Addresses PR #2029 review feedback.

Test Cases

1. test_semantic_get_dimension_mismatch_returns_none — store embedding with dim=3, query with dim=2 → None
2. test_semantic_get_dimension_mismatch_query_longer — store embedding with dim=2, query with dim=3 → None
3. test_semantic_get_mixed_dimensions_picks_correct_match — store both dim=2 and dim=3, query with dim=3 → matches only dim=3 entry (exact score ~1.0)
4. test_semantic_get_empty_embedding_skipped — empty embedding (BLOB x'') is scored 0.0, valid entry wins
5. test_semantic_get_corrupt_blob_skipped — corrupt BLOB (3 bytes) fails bytemuck deserialization, gracefully skipped
6. test_semantic_get_all_corrupt_returns_none — all candidates corrupt/empty → returns None

Key Implementation Notes

• Threshold value: All mismatch/empty tests use threshold=0.01 (not 0.0). This is critical because cosine_similarity() returns exactly 0.0 for dimension mismatches, and the cache logic uses score >= threshold. If threshold=0.0, then 0.0 >= 0.0 is true, producing a false cache HIT. Threshold > 0.0 correctly rejects mismatches.

• Test 3 score assertion: Verifies (score - 1.0).abs() < 1e-5 for identical embeddings to confirm cosine similarity returns ~1.0.

• Test isolation: All tests use in-memory SQLite (:memory:), no Qdrant, no testcontainers. Each test has an independent cache instance.

Validation Results

• Code review: Approved ✓
• Implementation critique: No blocking issues ✓
• Testing coverage: 6 tests cover all code paths in get_semantic() (dimension check, similarity filtering, corrupt BLOB handling) ✓
• Performance: 6 tests execute in ~42ms, no regressions ✓
• Security audit: No issues found ✓
• Full workspace tests: 5524 passed, 0 regressions ✓

Pre-merge checklist

• Formatting: cargo +nightly fmt --check passes
• Linting: cargo clippy --workspace -- -D warnings passes
• Tests: cargo nextest run --config-file .github/nextest.toml --workspace --lib --bins (5524 passed)
• No regressions
• All 6 required test cases implemented
• Commit message follows format

Related

• Closes test: add dimension mismatch deserialization test #2034
• References PR feat(memory): semantic response caching with embedding similarity #2029
• Related to semantic caching architecture spec (.local/specs/semantic-cache/spec.md)