fix(document-processors): make glyph-corruption ratio of 0 disable the signal

Address round-4 review on PR #914:
- glyph_corruption_ratio <= 0 now disables the signal (previously `control_ratio
  > 0` fired on any single C0 control byte), matching the "0 disables" convention
  used elsewhere (document_max_pdf_size_mb) and the config comment. Add a
  zero-disables test.
- Correct the document_escalation_suppressed_total comment: corrupt_glyphs CAN
  appear there in the narrow case where structured is unregistered and OCR is
  registered-but-disabled (evaluate_escalation follows minimum="structured" past
  the missing rung to a gated-off OCR). Add a test for that suppressed decision.
- Add a test for the double-corruption edge: a structured re-extract that is also
  glyph-corrupt escalates structured->ocr with reason corrupt_glyphs.

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

nextcloud_mcp_server/observability/metrics.py

@@ -287,9 +287,10 @@ document_escalation_total = Counter(
 document_escalation_suppressed_total = Counter(
     "astrolabe_document_escalation_suppressed_total",
     "Would-be tier escalations suppressed because the target tier is disabled",
-    # reason: low_confidence | empty_text. (corrupt_glyphs never appears here: it
-    # targets the structured tier, which has no enabled-gate -- if registered it
-    # runs, else there is nothing to suppress -- so it only ever hops or returns.)
+    # reason: low_confidence | empty_text | corrupt_glyphs. (corrupt_glyphs lands
+    # here only in the narrow case where the structured tier is unregistered AND
+    # OCR is registered-but-disabled: evaluate_escalation follows minimum="structured"
+    # past the missing rung to OCR, which is gated off -> suppressed{to_tier="ocr"}.)
     ["from_tier", "to_tier", "reason"],
 )