Add graph expansion and cross-encoder reranking features

- Implemented GraphExpander to enhance search results with related symbols using precomputed neighbors.
- Added CrossEncoderReranker for second-stage search ranking, allowing for improved result scoring.
- Created migrations to establish necessary database tables for relationships and graph neighbors.
- Developed tests for graph expansion functionality, ensuring related results are populated correctly.
- Enhanced performance benchmarks for cross-encoder reranking latency and graph expansion overhead.
- Updated schema cleanup tests to reflect changes in versioning and deprecated fields.
- Added new test cases for Treesitter parser to validate relationship extraction with alias resolution.

codex-lens/tests/test_search_comprehensive.py

@@ -582,6 +582,7 @@ class TestChainSearchResult:
         )
         assert result.query == "test"
         assert result.results == []
+        assert result.related_results == []
         assert result.symbols == []
         assert result.stats.dirs_searched == 0