gh-159658: Fix the REPL mangling input when the prompt is wider than the terminal - #159659

When the prompt fills the whole row, layout_content_lines() still forced one body character onto the prompt row, so the row overflowed the terminal and the first typed character got split from the rest. The force-progress fallback now only applies to rows without a prompt, so the prompt gets its own row and the input starts on the next one, the same as 3.14 does.

• Issue: _pyrepl: Mangled input when sys.ps1 is wider than terminal width #159658