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/document_processors/classifier.py

@@ -203,9 +203,15 @@ def _route_from_signals(
     Flags are diagnostic and independent of the verdict (e.g. ``image_heavy``
     fires on ANY image-heavy page; the OCR route needs a page FRACTION).
     """
-    # total_chars > 0 also guarantees this never overlaps the scanned branch
-    # (total_chars == 0), so a doc is never both glyph-corrupt and "scanned".
-    glyph_corrupt = total_chars > 0 and control_ratio > glyph_corruption_ratio
+    # glyph_corruption_ratio <= 0 disables the signal (a ratio of 0 would otherwise
+    # fire on any single C0 control byte). total_chars > 0 also guarantees this
+    # never overlaps the scanned branch (total_chars == 0), so a doc is never both
+    # glyph-corrupt and "scanned".
+    glyph_corrupt = (
+        glyph_corruption_ratio > 0
+        and total_chars > 0
+        and control_ratio > glyph_corruption_ratio
+    )
 
     flags: set[str] = set()
     if ocr_frac >= page_fraction and total_chars == 0: