Fix panic on RUF027 (#9990)
## Summary
Fixes #9895
The cause for this panic came from an offset error in the code. When
analyzing a hypothetical f-string, we attempt to re-parse it as an
f-string, and use the AST data to determine, among other things, whether
the format specifiers are correct. To determine the 'correctness' of a
format specifier, we actually have to re-parse the format specifier, and
this is where the issue lies. To get the source text for the specifier,
we were taking a slice from the original file source text... even though
the AST data for the specifier belongs to the standalone parsed f-string
expression, meaning that the ranges are going to be way off. In a file
with Unicode, this can cause panics if the slice is inside a char
boundary.
To fix this, we now slice from the temporary source we created earlier
to parse the literal as an f-string.
## Test Plan
The RUF027 snapshot test was amended to include a string with format
specifiers which we _should_ be calling out. This is to ensure we do
slice format specifiers from the source text correctly.
