Add test case for walrus operators in return types (#6438)

## Summary

Closes https://github.com/astral-sh/ruff/issues/6437.

## Test Plan

`cargo test`

crates/ruff_python_formatter/src/expression/expr_named_expr.rs

@@ -49,6 +49,7 @@ impl NeedsParentheses for ExprNamedExpr {
             || parent.is_expr_await()
             || parent.is_stmt_delete()
             || parent.is_stmt_for()
+            || parent.is_stmt_function_def()
         {
             OptionalParentheses::Always
         } else {