F-string debug mode does not print correctly when in last line of file

[lysnikolaou]

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 🐍 pyenv 3.11.3 took 15s 
❯ cat tmp/t.py
print(f'''{
3
=}''')% 

cpython on  main via C v14.0.3-clang via 🐍 pyenv 3.11.3 
❯ ./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.

Linked PRs

• gh-105831: Fix NEWS blurb from gh-105828 #105833