Formatter quoting for f-strings with triple quotes (#7826)

**Summary** Quoting of f-strings can change if they are triple quoted
and only contain single quotes inside.

Fixes #6841

**Test Plan** New fixtures

---------

Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/fstring.py

@@ -57,3 +57,8 @@ result_f = (
     ),
     2
 )
+
+# https://github.com/astral-sh/ruff/issues/6841
+x = f'''a{""}b'''
+y = f'''c{1}d"""e'''
+z = f'''a{""}b''' f'''c{1}d"""e'''

crates/ruff_python_formatter/src/expression/string.rs

@@ -48,10 +48,19 @@ impl<'a> AnyString<'a> {
         match self {
             Self::Constant(_) => Quoting::CanChange,
             Self::FString(f_string) => {
+                let unprefixed = locator
+                    .slice(f_string.range)
+                    .trim_start_matches(|c| c != '"' && c != '\'');
+                let triple_quoted =
+                    unprefixed.starts_with(r#"""""#) || unprefixed.starts_with(r#"'''"#);
                 if f_string.values.iter().any(|value| match value {
                     Expr::FormattedValue(ast::ExprFormattedValue { range, .. }) => {
                         let string_content = locator.slice(*range);
-                        string_content.contains(['"', '\''])
+                        if triple_quoted {
+                            string_content.contains(r#"""""#) || string_content.contains("'''")
+                        } else {
+                            string_content.contains(['"', '\''])
+                        }
                     }
                     _ => false,
                 }) {

crates/ruff_python_formatter/tests/snapshots/format@expression__fstring.py.snap

@@ -63,6 +63,11 @@ result_f = (
     ),
     2
 )
+
+# https://github.com/astral-sh/ruff/issues/6841
+x = f'''a{""}b'''
+y = f'''c{1}d"""e'''
+z = f'''a{""}b''' f'''c{1}d"""e'''
 ```
 
 ## Output
@@ -124,6 +129,11 @@ result_f = (
     ),
     2,
 )
+
+# https://github.com/astral-sh/ruff/issues/6841
+x = f"""a{""}b"""
+y = f'''c{1}d"""e'''
+z = f"""a{""}b""" f'''c{1}d"""e'''
 ```