fix(documents): use strip_pdf_content_marker instead of lstrip for PDF auto-open (#1727)

lstrip("\n[PDF content]:") treats the argument as a character set,
not a prefix, so it chews into the following [Page N text]: marker —
e.g. turning [Page 1 text]: into "age 1 text]:". The correct helper
strip_pdf_content_marker (which uses removeprefix) already exists in
the same file and is used by other call sites.

Fixes #1663

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/document_processor.py

@@ -394,9 +394,7 @@ def build_user_content(
                         # Pull the PDF prose once — used as either intro_text
                         # (form path) or the doc body (plain path).
                         try:
-                            pdf_body_text = _process_pdf(path).lstrip(
-                                "\n[PDF content]:"
-                            ).strip()
+                            pdf_body_text = strip_pdf_content_marker(_process_pdf(path))
                         except Exception:
                             pdf_body_text = None