When fixing an invalid escape sequence in an f-string, each f-string
element is analyzed for valid escape characters prior to creating the
diagnostic and fix. This allows us to safely prefix with `r` to create a
raw string if no valid escape characters were found anywhere in the
f-string, and otherwise insert backslashes.
This fixes a bug in the original implementation: each "f-string part"
was treated separately, so it was not possible to tell whether a valid
escape character was or would be used elsewhere in the f-string.
Progress towards #11491 but format specifiers are not handled in this
PR.
## Summary
This PR fixes a panic in the linter for `W605`.
Consider the following f-string:
```python
f"{{}}ab"
```
The `FStringMiddle` token would contain `{}ab`. Notice that the escaped
braces have _reduced_ the string. This means we cannot use the text
value from the token to determine the location of the escape sequence
but need to extract it from the source code.
fixes: #10434
## Test Plan
Add new test cases and update the snapshots.
## Summary
If a string has a Unicode prefix, we can't add the `r` prefix on top of
that -- we need to remove and replace it. (The Unicode prefix is
redundant anyway in Python 3.)
Closes https://github.com/astral-sh/ruff/issues/8967.
