Ignore trailing quotes for unclosed l-brace errors (#9388)
## Summary
Given:
```python
F"{"ڤ
```
We try to locate the "unclosed left brace" error by subtracting the
quote size from the lexer offset -- so we subtract 1 from the end of the
source, which puts us in the middle of a Unicode character. I don't
think we should try to adjust the offset in this way, since there can be
content _after_ the quote. For example, with the advent of PEP 701, this
string could reasonably be fixed as:
```python
F"{"ڤ"}"
````
Closes https://github.com/astral-sh/ruff/issues/9379.
