string-processing: f-strings with quoted debug expressions

[MichaReiser]

Describe the bug

The string-processing produces an invalid f-string when joining together two strings where both literals have a debug expression with quotes

f"{10 + len('bar')=}" f'{10 + len("bar")=}'

Cannot parse for target version Python 3.13: 2:30: f"{10 + len('bar')=}{10 + len(\"bar\")=}"

To Reproduce

Playground

Expected behavior

Black shouldn't join f-strings containing debug expressions with different quotes because changing the quotes would change the runtime behavior (the printed expressions use different quotes)

[krtimisra67]

f"{10 + len('bar')=}{10 + len('bar')=}"
or
f'{10 + len("bar")=}{10 + len("bar")=}'

[cobaltt7]

fixed by #4498