Implement ANN index using HNSW algorithm and update related tests

- Added ANNIndex class for approximate nearest neighbor search using HNSW.
- Integrated ANN index with VectorStore for enhanced search capabilities.
- Updated test suite for ANN index, including tests for adding, searching, saving, and loading vectors.
- Modified existing tests to accommodate changes in search performance expectations.
- Improved error handling for file operations in tests to ensure compatibility with Windows file locks.
- Adjusted hybrid search performance assertions for increased stability in CI environments.

codex-lens/pyproject.toml

@@ -28,6 +28,7 @@ dependencies = [
 semantic = [
     "numpy>=1.24",
     "fastembed>=0.2",
+    "hnswlib>=0.8.0",
 ]
 
 # Encoding detection for non-UTF8 files