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

python/ebook_search/eval/dataset.py

@@ -28,11 +28,11 @@ class GoldQuery:
 
 
 def load_gold_queries(path: Path = DEFAULT_QUERIES_PATH) -> list[GoldQuery]:
-    """Load labeled queries from a JSONL file."""
+    """Load labeled queries from a JSONL file. Blank lines and ``//`` comment lines are skipped."""
     queries: list[GoldQuery] = []
     for line in path.read_text(encoding="utf-8").splitlines():
         stripped = line.strip()
-        if not stripped:
+        if not stripped or stripped.startswith("//"):
             continue
         record = json.loads(stripped)
         queries.append(