fix(document): apply OCR timeout to Mistral backend + review/Sonar fixes (#892)

Round-1 review on PR #892:
- Wire DOCUMENT_OCR_TIMEOUT_SECONDS into _MistralOcrBackend too (was
  gateway-only): wrap process_async in anyio.fail_after so the SDK-managed
  client honours the setting; on expiry it fails fast as a clean parse error.
  Test added.
- Tighten the misleading "honoured without a restart" comment — per-call
  get_settings() is for test monkeypatching; a live change still needs a
  restart since the backend is cached for the pod lifetime.
- Comment the size guard's two intentional gaps: an explicit processor_name
  override bypasses it, and the early return skips the parse-duration histogram.

SonarCloud (new-code smells in the added tests):
- S1244 float-equality asserts → pytest.approx (test_config.py, test_ocr_processor.py).
- S1186/S7503: rewrite the gateway-timeout test with mocker AsyncMock/MagicMock
  instead of a hand-rolled fake client (no empty method, no async-without-await).

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

nextcloud_mcp_server/document_processors/registry.py

@@ -205,7 +205,13 @@ class ProcessorRegistry:
         # Pre-parse size guard: a pathologically large PDF (e.g. a 42 MB scanned
         # DUDE) burns the OCR timeout for 0 chars. Fail fast with an explicit
         # reason so the caller marks the placeholder "failed" instead of
-        # retrying. 0 disables the cap.
+        # retrying. 0 disables the cap. This lives on the auto-tiered path only:
+        # an explicit processor_name="ocr" override (registry.process) bypasses
+        # _process_pdf entirely and is intentionally not size-gated (power-user
+        # escape hatch). Returning here also skips _run_processor, so the
+        # rejection is counted on astrolabe_document_parse_failed_total{oversize}
+        # (via vector/processor.py) but deliberately not on the parse-duration
+        # histogram -- there is no parse to time.
         max_pdf_mb = settings.document_max_pdf_size_mb
         if max_pdf_mb > 0 and len(content) > max_pdf_mb * 1024 * 1024:
             size_mb = len(content) / (1024 * 1024)