Fix fstring formatting removing overlong implicit concatenated string in expression part

[MichaReiser]

Summary

Fixes #14778

The formatter incorrectly removed the inner implicitly concatenated string for following single-line f-string:

f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"

# formatted
f"{ if True else ''}"

This happened because I changed the RemoveSoftlinesBuffer in #14489 to remove any content wrapped in if_group_breaks. After all, it emulates an all flat layout. This works fine when if_group_breaks is only used to add content if the gorup breaks. It doesn't work if the same content is rendered differently depending on if the group fits using if_group_breaks and if_groups_fits because the enclosing group might still break if the entire content exceeds the line-length limit.

This PR fixes this by unwrapping any if_group_fits content by removing the if_group_fits start and end tags.

Test Plan

added test

[github-actions]

ruff-ecosystem results

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[dhruvmanila]

Thanks