Avoid moving back the lexer for triple-quoted fstring (#11939)

## Summary

This PR avoids moving back the lexer for a triple-quoted f-string during
the re-lexing phase.

The reason this is a problem is that for a triple-quoted f-string the
newlines are part of the f-string itself, specifically they'll be part
of the `FStringMiddle` token. So, if we moved the lexer back, there
would be a `Newline` token whose range would be in between an
`FStringMiddle` token. This creates a panic in downstream usage.

fixes: #11937 

## Test Plan

Add test cases and validate the snapshots.

crates/ruff_python_parser/src/lexer.rs

@@ -1370,6 +1370,12 @@ impl<'src> Lexer<'src> {
         // i.e., it recovered from an unclosed parenthesis (`(`, `[`, or `{`).
         self.nesting -= 1;
 
+        // The lexer can't be moved back for a triple-quoted f-string because the newlines are
+        // part of the f-string itself, so there is no newline token to be emitted.
+        if self.current_flags.is_triple_quoted_fstring() {
+            return false;
+        }
+
         let mut current_position = self.current_range().start();
         let reverse_chars = self.source[..current_position.to_usize()].chars().rev();
         let mut newline_position = None;
@@ -1578,6 +1584,11 @@ impl TokenFlags {
         self.intersects(TokenFlags::F_STRING)
     }
 
+    /// Returns `true` if the token is a triple-quoted f-string.
+    fn is_triple_quoted_fstring(self) -> bool {
+        self.contains(TokenFlags::F_STRING | TokenFlags::TRIPLE_QUOTED_STRING)
+    }
+
     /// Returns `true` if the token is a raw string.
     const fn is_raw_string(self) -> bool {
         self.intersects(TokenFlags::RAW_STRING)