fix(evidence): fail closed when BM25 is unavailable

harness/tests/l0/test_qdrant_bm25_inference.py

@@ -3,7 +3,9 @@
 from __future__ import annotations
 
 import time
+from contextlib import ExitStack
 from pathlib import Path
+from uuid import uuid4
 
 import pytest
 import requests
@@ -59,10 +61,10 @@ def dense_result_id(base_url: str, collection: str, query: list[float]) -> int:
 
 
 def test_pinned_qdrant_image_indexes_and_queries_italian_bm25_server_side():
-    with DockerContainer(qdrant_image()).with_exposed_ports(6333) as qdrant:
+    with DockerContainer(qdrant_image()).with_exposed_ports(6333) as qdrant, ExitStack() as cleanup:
         base_url = f"http://{qdrant.get_container_host_ip()}:{qdrant.get_exposed_port(6333)}"
         wait_for_qdrant(base_url)
-        collection = "italian_bm25_contract"
+        collection = f"italian_bm25_contract_{uuid4().hex}"
         request_ok(
             "PUT",
             f"{base_url}/collections/{collection}",
@@ -70,6 +72,7 @@ def test_pinned_qdrant_image_indexes_and_queries_italian_bm25_server_side():
                 "vectors": {"size": 4, "distance": "Cosine"},
             },
         )
+        cleanup.callback(request_ok, "DELETE", f"{base_url}/collections/{collection}")
         legacy_points = [
             {"id": 10, "vector": [1.0, 0.0, 0.0, 0.0], "payload": {"record_kind": "schema_table"}},
             {"id": 11, "vector": [0.0, 1.0, 0.0, 0.0], "payload": {"record_kind": "schema_column"}},