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_rerank.py

@@ -33,7 +33,7 @@ def rerank_response(payload: dict[str, object] | None = None, *, content: bytes
     )
 
 
-def test_config_defaults_keep_reranking_optional(mocker: MockerFixture) -> None:
+def test_config_defaults_enable_reranking(mocker: MockerFixture) -> None:
     mocker.patch.dict(environ, {}, clear=False)
     environ.pop("EBOOK_SEARCH_RERANK_ENABLED", None)
     environ.pop("EBOOK_SEARCH_RERANK_BASE_URL", None)
@@ -43,7 +43,7 @@ def test_config_defaults_keep_reranking_optional(mocker: MockerFixture) -> None:
 
     config = load_rerank_config()
 
-    assert config.enabled is False
+    assert config.enabled is True
     assert config.base_url == "http://192.168.90.25:8001"
     assert config.model == "qwen3-reranker-06b"
     assert config.candidates == 24