F-string debug mode does not print correctly when in last line of file
Where there's an f-string that has a debug expression in the last line of a file, the debug expression buffer is one character too short. For example:
```python3 cpython on ๎ main via C v14.0.3-clang via ๐ pyenv 3.11.3 took 15s โฏ cat tmp/t.py print(f'''{ 3 =}''')% cpython on ๎ main via C v14.0.3-clang via ๐ pyenv 3.11.3 โฏ ./python.exe tmp/t.py 3 3
A fix is already up as part of gh-105828, so just opening this to track the issue.