Extract Pages: Auto-append .pdf extension (align with Rename behavior)

The Extract Pages action’s “New File Name” field requires the user to manually type “.pdf” — omitting it returns: “New file [name] has invalid file name.” The plain Rename action does not require the extension and handles it correctly. This inconsistency within CLM.DS itself is a friction point. Request: either auto-append the extension on split, or align Extract Pages validation with Rename so users don’t need to remember which action requires what.