Fix manual naive parser position extraction fallback (#14420)

### What problem does this PR solve?
This PR fixes a regression where Manual pipeline + Naive (Plain Text)
PDF parsing crashed with `AttributeError: 'PlainParser' object has no
attribute 'extract_positions'` in `rag/app/manual.py`.
fixes #14411 
### Type of change:
- [x] Bug Fix (non-breaking change which fixes an issue)

rag/app/manual.py

@@ -183,7 +183,7 @@ def chunk(filename, binary=None, from_page=0, to_page=MAXIMUM_PAGE_NUMBER, lang=
 
             txt, layoutno, poss = section
             if isinstance(poss, str):
-                poss = pdf_parser.extract_positions(poss)
+                poss = (getattr(pdf_parser, "extract_positions", lambda _: [])(poss) or [[0, 0, 0, 0, 0]])
                 if poss:
                     first = poss[0]  # tuple: ([pn], x1, x2, y1, y2)
                     pn = first[0]