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.rs

@@ -700,7 +700,7 @@ impl<'source> Lexer<'source> {
                 }
                 Some('\r' | '\n') if !triple_quoted => {
                     if let Some(fstring) = self.fstrings.current() {
-                        // When we are in an f-string, check whether does the initial quote
+                        // When we are in an f-string, check whether the initial quote
                         // matches with f-strings quotes and if it is, then this must be a
                         // missing '}' token so raise the proper error.
                         if fstring.quote_char() == quote && !fstring.is_triple_quoted() {
@@ -708,7 +708,7 @@ impl<'source> Lexer<'source> {
                                 error: LexicalErrorType::FStringError(
                                     FStringErrorType::UnclosedLbrace,
                                 ),
-                                location: self.offset() - fstring.quote_size(),
+                                location: self.offset() - TextSize::new(1),
                             });
                         }
                     }
@@ -732,7 +732,7 @@ impl<'source> Lexer<'source> {
                 Some(_) => {}
                 None => {
                     if let Some(fstring) = self.fstrings.current() {
-                        // When we are in an f-string, check whether does the initial quote
+                        // When we are in an f-string, check whether the initial quote
                         // matches with f-strings quotes and if it is, then this must be a
                         // missing '}' token so raise the proper error.
                         if fstring.quote_char() == quote
@@ -742,7 +742,7 @@ impl<'source> Lexer<'source> {
                                 error: LexicalErrorType::FStringError(
                                     FStringErrorType::UnclosedLbrace,
                                 ),
-                                location: self.offset() - fstring.quote_size(),
+                                location: self.offset(),
                             });
                         }
                     }
@@ -2195,13 +2195,17 @@ f"{(lambda x:{x})}"
         assert_debug_snapshot!(lex_jupyter_source(source));
     }
 
+    fn lex_error(source: &str) -> LexicalError {
+        match lex(source, Mode::Module).find_map(Result::err) {
+            Some(err) => err,
+            _ => panic!("Expected at least one error"),
+        }
+    }
+
     fn lex_fstring_error(source: &str) -> FStringErrorType {
-        match lex(source, Mode::Module).find_map(std::result::Result::err) {
-            Some(err) => match err.error {
-                LexicalErrorType::FStringError(error) => error,
-                _ => panic!("Expected FStringError: {err:?}"),
-            },
-            _ => panic!("Expected atleast one FStringError"),
+        match lex_error(source).error {
+            LexicalErrorType::FStringError(error) => error,
+            err => panic!("Expected FStringError: {err:?}"),
         }
     }
 
@@ -2246,4 +2250,25 @@ f"{(lambda x:{x})}"
             UnterminatedTripleQuotedString
         );
     }
+
+    #[test]
+    fn test_fstring_error_location() {
+        assert_debug_snapshot!(lex_error("f'{'"), @r###"
+        LexicalError {
+            error: FStringError(
+                UnclosedLbrace,
+            ),
+            location: 4,
+        }
+        "###);
+
+        assert_debug_snapshot!(lex_error("f'{'α"), @r###"
+        LexicalError {
+            error: FStringError(
+                UnclosedLbrace,
+            ),
+            location: 6,
+        }
+        "###);
+    }
 }

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> {