test(usage): close round-5 nits (empty doc_types, consistency tidy-ups)

Round-5 claude-review (merge-ready; all nits):

- 🟡 Added test_empty_doc_types_normalizes_to_null pinning doc_types=[] → None
  in record_search_usage metadata (matches the None case).
- 🟡 record_search_usage docstring now notes nc_semantic_search_answer always
  meters with doc_types=None (it exposes no doc_types parameter).
- 🟢 BM25HybridSearchAlgorithm.__init__ now sets query_embedding /
  query_token_count alongside _embedded_query, so all three cache fields are
  instance attributes from construction (was relying on the class-level
  SearchAlgorithm defaults).
- 🟢 Ollama embed_batch_with_usage caches _dimension inline (mirrors
  OpenAI/Mistral), so the dimension is set via any embed path.
- 🟢 record_indexing_usage documents the independent-record / partial-failure
  semantics under SUM aggregation.

Deck #67.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/processor.py

@@ -153,6 +153,10 @@ async def record_indexing_usage(
         store = await UsageEventStore.shared()
         # enabled=True: the guard above already confirmed the flag, so the store
         # skips a second uncached Settings build per record (ADR-024).
+        # record_usage_event swallows its own write failures, so the two records
+        # are independent; if pages_chunks somehow raised mid-way, embeddings_-
+        # queries would be skipped, leaving an unmatched pages_chunks row —
+        # acceptable under the (day, metric) SUM-aggregation billing model.
         await store.record_usage_event(
             metric="pages_chunks", value=chunk_count, metadata=metadata, enabled=True
         )