fix(ebook-search): skip comment lines in gold query loader and realign tests

load_gold_queries now skips blank and `//` comment lines so the committed
section separator in queries.jsonl no longer breaks dataset/load-test loading.

Update tests left stale by the search refactor (6bc3011):
- pass the now-required rank_constant to reciprocal_rank_fusion
- expect bm25_candidates to receive the full query and drop the removed
  "BM25 query preparation" timing step
- assert reranking is enabled by default

tests/ebook_search/test_rag_pipeline.py

@@ -30,7 +30,7 @@ def test_search_ebooks_runs_vector_and_bm25_in_parallel(mocker: MockerFixture) -
 
     def fake_bm25_candidates(query, _config):
         """Return BM25 candidates after confirming vector search has started."""
-        assert query == "parallel"
+        assert query == "what is parallel"
         bm25_started.set()
         assert vector_started.wait(timeout=2)
         return [SearchResult(chunk_id=2, text="bm25", source_title="BM25", bm25_score=2.0)]
@@ -46,5 +46,4 @@ def test_search_ebooks_runs_vector_and_bm25_in_parallel(mocker: MockerFixture) -
     assert timings["Embedding + vector search"].counts_toward_total is False
     assert timings["BM25 search"].counts_toward_total is False
     assert timings["Hybrid retrieval"].counts_toward_total is True
-    assert timings["BM25 query preparation"].counts_toward_total is True
     assert received_engines == [engine]