Namely, if such composed literal has nodes with different quotes, convert only
the one directly under the caret. This way we both preserve the old behavior,
when quotes of all individual strings of the literal were converted, and allow
to convert quotes of at least one element if that is not possible.
I've also migrated the intention to newer PyStringElement API that appeared with
the new f-string support.
GitOrigin-RevId: a8592a46e2991b172a205f041cbc3b668e242e8a
