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/document_processors/base.py

@@ -2,7 +2,7 @@
 
 from abc import ABC, abstractmethod
 from collections.abc import Awaitable, Callable
-from typing import Any, Optional
+from typing import Any
 
 from pydantic import BaseModel
 
@@ -22,7 +22,7 @@ class ProcessingResult(BaseModel):
     success: bool = True
     """Whether processing succeeded"""
 
-    error: Optional[str] = None
+    error: str | None = None
     """Error message if processing failed"""
 
 
@@ -83,11 +83,10 @@ class DocumentProcessor(ABC):
         self,
         content: bytes,
         content_type: str,
-        filename: Optional[str] = None,
-        options: Optional[dict[str, Any]] = None,
-        progress_callback: Optional[
-            Callable[[float, Optional[float], Optional[str]], Awaitable[None]]
-        ] = None,
+        filename: str | None = None,
+        options: dict[str, Any] | None = None,
+        progress_callback: Callable[[float, float | None, str | None], Awaitable[None]]
+        | None = None,
     ) -> ProcessingResult:
         """Process a document and extract text.