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.

crates/ruff_python_parser/src/lexer/fstring.rs

@@ -1,7 +1,5 @@
 use bitflags::bitflags;
 
-use ruff_text_size::TextSize;
-
 bitflags! {
     #[derive(Debug)]
     pub(crate) struct FStringContextFlags: u8 {
@@ -58,15 +56,6 @@ impl FStringContext {
         }
     }
 
-    /// Returns the number of quotes for the current f-string.
-    pub(crate) const fn quote_size(&self) -> TextSize {
-        if self.is_triple_quoted() {
-            TextSize::new(3)
-        } else {
-            TextSize::new(1)
-        }
-    }
-
     /// Returns the triple quotes for the current f-string if it is a triple-quoted
     /// f-string, `None` otherwise.
     pub(crate) const fn triple_quotes(&self) -> Option<&'static str> {