fix(observability): address second review round

- Failed deletes no longer bump astrolabe_documents_indexed_total: the outer
  except in process_document now gates doc_type on operation != "delete", so a
  delete error is counted as processed-error but not as an indexing event.
  Added test_failed_delete_is_processed_but_not_indexed.
- registry parse span: pass record_exception=True explicitly (matches
  instrument_tool) and add a structured logger.warning on the parse-error path
  (processor/tier/byte_size/duration_ms) for a Loki-aggregatable failed-parse
  signal.
- test_error_does_not_increment_throughput: snapshot-before/delta pattern
  instead of absolute 0.0 (counters are global singletons).
- config: document the deliberate gateway asymmetry between
  get_embedding_model_name() (no gateway branch) and
  get_embedding_provider_family() (short-circuits on gateway).
- Cleanup in touched scope: narrow `except (HTTPStatusError, Exception)` to
  `except Exception` (drop now-unused import); convert registry signatures from
  Optional[...] to `... | None`.

Refs Deck #175, PR #831.

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

nextcloud_mcp_server/config.py

@@ -931,6 +931,12 @@ class Settings:
         Returns:
             Active embedding model name
         """
+        # NOTE: there is intentionally no "gateway" branch here. When
+        # EMBEDDING_PROVIDER=gateway this falls through to the underlying
+        # provider's model (used for the Qdrant collection name), whereas
+        # get_embedding_provider_family() short-circuits to the gateway-routed
+        # family. Keep that asymmetry in mind before joining metrics/labels
+        # derived from these two methods.
         if (
             self.aws_region
             or self.bedrock_embedding_model