## 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.