fix(ingest): address review round 2 (Literal reason + exhaustive branch + test)

- escalation: EscalationDecision.reason is now Literal["empty_text",
  "low_confidence"] (parity with kind; ty catches a bad label at call sites).
- processor: nest the decision handling so the hop branch is reached via an
  explicit else under `if decision is not None` — exhaustive over the Literal
  kind, no None-attribute risk.
- tests: add the "OCR processor unregistered (not just disabled) → None"
  quadrant, locking in absent != suppressed.

Deck #324.

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

nextcloud_mcp_server/document_processors/escalation.py

@@ -49,7 +49,7 @@ class EscalationDecision:
 
     kind: Literal["hop", "suppressed"]
     to_tier: str
-    reason: str  # empty_text | low_confidence
+    reason: Literal["empty_text", "low_confidence"]
 
 
 def next_tier(current: str) -> str | None: