Fix formatting of trailing unescaped quotes in raw triple quoted strings (#6202)

**Summary** This prevents us from turning `r'''\""'''` into
`r"""\"""""`, which is invalid syntax.

This PR fixes CI, which is currently broken on main (in a way that still
passes on linter PRs and allows merging formatter PRs, but it's bad to
have a job be red). Once merged, i'll make the formatted ecosystem
checks a required check.

**Test Plan** Added a regression test.

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

@@ -24,6 +24,12 @@ U"Test"
 r"Test"
 R"Test"
 
+# Block conversion if there is an unescaped quote just before the end of the triple
+# quoted string
+r'''\""'''
+r'''""'''
+r'\""'
+
 'This string will not include \
 backslashes or newline characters.'
 

crates/ruff_python_formatter/src/expression/string.rs

@@ -323,15 +323,22 @@ fn preferred_quotes_raw(
                     break true;
                 }
 
-                if chars.peek() == Some(&configured_quote_char) {
+                match chars.peek() {
+                    // We can't turn `r'''\""'''` into `r"""\"""""`, this would confuse the parser
+                    // about where the closing triple quotes start
+                    None => break true,
+                    Some(next) if *next == configured_quote_char => {
                         // `""` or `''`
                         chars.next();
 
-                    if chars.peek() == Some(&configured_quote_char) {
-                        // `"""` or `'''`
+                        // We can't turn `r'''""'''` into `r""""""""`, nor can we have
+                        // `"""` or `'''` respectively inside the string
+                        if chars.peek().is_none() || chars.peek() == Some(&configured_quote_char) {
                             break true;
                         }
                     }
+                    _ => {}
+                }
             }
             Some(_) => continue,
             None => break false,

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

@@ -30,6 +30,12 @@ U"Test"
 r"Test"
 R"Test"
 
+# Block conversion if there is an unescaped quote just before the end of the triple
+# quoted string
+r'''\""'''
+r'''""'''
+r'\""'
+
 'This string will not include \
 backslashes or newline characters.'
 
@@ -162,6 +168,12 @@ magic-trailing-comma    = Respect
 r"Test"
 R"Test"
 
+# Block conversion if there is an unescaped quote just before the end of the triple
+# quoted string
+r'''\""'''
+r'''""'''
+r'\""'
+
 "This string will not include \
 backslashes or newline characters."
 
@@ -310,6 +322,12 @@ magic-trailing-comma    = Respect
 r'Test'
 R'Test'
 
+# Block conversion if there is an unescaped quote just before the end of the triple
+# quoted string
+r'''\""'''
+r'''""'''
+r'\""'
+
 'This string will not include \
 backslashes or newline characters.'