test: Add test suites for embedding, FTS, hybrid search, and golden queries

Four new test modules covering the search infrastructure:

- tests/embedding.rs: Unit tests for the embedding pipeline including
  chunk ID encoding/decoding, change detection, and document chunking
  with overlap verification.

- tests/fts_search.rs: Integration tests for FTS5 search including
  safe query sanitization, multi-term queries, prefix matching, and
  the raw FTS mode for power users.

- tests/hybrid_search.rs: End-to-end tests for hybrid search mode
  including RRF fusion correctness, graceful degradation when
  embeddings are unavailable, and filter application.

- tests/golden_query_tests.rs: Golden query tests using fixtures
  from tests/fixtures/golden_queries.json to verify search quality
  against known-good query/result pairs. Ensures ranking stability
  across implementation changes.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/fixtures/golden_queries.json

@@ -0,0 +1,65 @@
+[
+  {
+    "query": "authentication login",
+    "mode": "lexical",
+    "filters": {},
+    "expected_doc_ids": [1],
+    "min_results": 1,
+    "max_rank": 10,
+    "description": "Basic auth keywords should find the OAuth login issue"
+  },
+  {
+    "query": "database migration",
+    "mode": "lexical",
+    "filters": {},
+    "expected_doc_ids": [3],
+    "min_results": 1,
+    "max_rank": 10,
+    "description": "Database migration terms should find the migration issue"
+  },
+  {
+    "query": "user profile",
+    "mode": "lexical",
+    "filters": {},
+    "expected_doc_ids": [2],
+    "min_results": 1,
+    "max_rank": 10,
+    "description": "User profile keywords should find the profile MR"
+  },
+  {
+    "query": "API rate limiting",
+    "mode": "lexical",
+    "filters": {},
+    "expected_doc_ids": [5],
+    "min_results": 1,
+    "max_rank": 10,
+    "description": "Rate limiting query should find the discussion document"
+  },
+  {
+    "query": "performance optimization",
+    "mode": "lexical",
+    "filters": {},
+    "expected_doc_ids": [4],
+    "min_results": 1,
+    "max_rank": 10,
+    "description": "Performance terms should find the performance MR"
+  },
+  {
+    "query": "token refresh",
+    "mode": "lexical",
+    "filters": {"source_type": "issue"},
+    "expected_doc_ids": [1],
+    "min_results": 1,
+    "max_rank": 10,
+    "description": "Token refresh with issue filter should find auth issue only"
+  },
+  {
+    "query": "CSS styling frontend",
+    "mode": "lexical",
+    "filters": {},
+    "expected_doc_ids": [6],
+    "min_results": 1,
+    "max_rank": 10,
+    "description": "Frontend CSS query should find the UI improvements issue"
+  }
+]