refactor(ocr): make collapse_adjacent_markers a public function

Drop underscore prefix — the helper is part of confidence.py's effective
public API since spell_check.py imports and calls it directly.

Fixes reviewer concern: importing a _-prefixed name across module boundaries
contradicts Python's private-by-convention signal.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ocr-service/spell_check.py

@@ -5,7 +5,7 @@ import os
 
 from spellchecker import SpellChecker
 
-from confidence import CORRECTION_MARKER, ILLEGIBLE_MARKER, _collapse_adjacent_markers
+from confidence import CORRECTION_MARKER, ILLEGIBLE_MARKER, collapse_adjacent_markers
 
 logger = logging.getLogger(__name__)
 
@@ -114,4 +114,4 @@ def correct_text(text: str) -> str:
         else:
             checked.append(ILLEGIBLE_MARKER)
 
-    return " ".join(_collapse_adjacent_markers(checked))
+    return " ".join(collapse_adjacent_markers(checked))