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/resources/test/fixtures/ruff/statement/function.py

@@ -371,3 +371,8 @@ def f(  # first
     # third
 ):
     ...
+
+# Walrus operator in return type.
+def this_is_unusual() -> (please := no): ...
+
+def this_is_unusual(x) -> (please := no): ...

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 {

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

@@ -377,6 +377,11 @@ def f(  # first
     # third
 ):
     ...
+
+# Walrus operator in return type.
+def this_is_unusual() -> (please := no): ...
+
+def this_is_unusual(x) -> (please := no): ...
 ```
 
 ## Output
@@ -905,6 +910,15 @@ def f(  # first
     /,  # second
 ):
     ...
+
+
+# Walrus operator in return type.
+def this_is_unusual() -> (please := no):
+    ...
+
+
+def this_is_unusual(x) -> (please := no):
+    ...
 ```