fix(observability): address third review round

Remaining items from the PR #831 Claude review:

- processor span symmetry: add "vector_sync.total_chars" to the sparse
  embedding span (already on the dense span) and drop the redundant
  "embedding.batch_size" attribute from both spans — it always equalled
  vector_sync.chunk_count and would mislead once batching is split.
- metrics: document the deliberate "throughput counts only on full success"
  contract in record_document_parse (partial extractions flagged
  success=False are counted as a parse-error but never inflate
  pages/chars/bytes throughput).
- config: extract _detect_base_provider() -> (family, model) as the single
  source of truth for the provider-detection priority chain, shared by
  get_embedding_model_name() and get_embedding_provider_family(). Preserves
  the intentional gateway asymmetry (only the family method short-circuits).
- base.py: Optional[...] -> PEP 604 `... | None`; drop now-unused import.

Behavior unchanged (get_embedding_* outputs covered by test_config.py).

Refs Deck #175, PR #831.

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

nextcloud_mcp_server/observability/metrics.py

@@ -533,6 +533,10 @@ def record_document_parse(
         processor=processor, tier=tier, status=status
     ).observe(duration)
     document_parse_total.labels(processor=processor, tier=tier, status=status).inc()
+    # Throughput counters (pages/chars/bytes) accrue only on a full success.
+    # A partial extraction flagged success=False is recorded above as a
+    # parse-error but is intentionally excluded here so low-confidence output
+    # never inflates pipeline throughput.
     if status == "success":
         if pages > 0:
             document_pages_processed_total.labels(processor=processor, tier=tier).inc(