Place 'r' prefix before 'f' for raw format strings (#8464)

## Summary

Currently, `UP032` applied to raw strings results in format strings with
the prefix 'fr'. This gets changed to 'rf' by Ruff format (or Black). In
order to avoid that, this PR uses the prefix 'rf' to begin with.

## Test Plan

Updated the expectation on an existing test.

crates/ruff_linter/src/rules/pyupgrade/rules/f_strings.rs

@@ -191,15 +191,21 @@ fn try_convert_to_f_string(
     summary: &mut FormatSummaryValues,
     locator: &Locator,
 ) -> Result<Option<String>> {
+    let contents = locator.slice(range);
+
     // Strip the unicode prefix. It's redundant in Python 3, and invalid when used
     // with f-strings.
-    let contents = locator.slice(range);
     let contents = if contents.starts_with('U') || contents.starts_with('u') {
         &contents[1..]
     } else {
         contents
     };
 
+    // Temporarily strip the raw prefix, if present. It will be prepended to the result, before the
+    // 'f', to match the prefix order both the Ruff formatter (and Black) use when formatting code.
+    let raw = contents.starts_with('R') || contents.starts_with('r');
+    let contents = if raw { &contents[1..] } else { contents };
+
     // Remove the leading and trailing quotes.
     let leading_quote = leading_quote(contents).context("Unable to identify leading quote")?;
     let trailing_quote = trailing_quote(contents).context("Unable to identify trailing quote")?;
@@ -291,7 +297,10 @@ fn try_convert_to_f_string(
     }
 
     // Construct the format string.
-    let mut contents = String::with_capacity(1 + converted.len());
+    let mut contents = String::with_capacity(usize::from(raw) + 1 + converted.len());
+    if raw {
+        contents.push('r');
+    }
     contents.push('f');
     contents.push_str(leading_quote);
     contents.push_str(&converted);

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP032_0.py.snap

@@ -353,7 +353,7 @@ UP032_0.py:37:1: UP032 [*] Use f-string instead of `format` call
 35 35 | "foo{}".format(1)
 36 36 | 
 37    |-r"foo{}".format(1)
-   37 |+fr"foo{1}"
+   37 |+rf"foo{1}"
 38 38 | 
 39 39 | x = "{a}".format(a=1)
 40 40 |