Insert trailing comma when function breaks with single argument (#8921)

## Summary

Given:

```python
def _example_function_xxxxxxx(
    variable: Optional[List[str]]
) -> List[example.ExampleConfig]:
    pass
```

We should be inserting a trailing comma after the argument (as long as
it's a single-argument function). This was an inconsistency with Black,
but also led to some internal inconsistencies, whereby we added the
comma if the argument contained a trailing end-of-line comment, but not
otherwise.

Closes https://github.com/astral-sh/ruff/issues/8912.

## Test Plan

`cargo test`

Before:

| project | similarity index | total files | changed files |

|----------------|------------------:|------------------:|------------------:|
| cpython | 0.75804 | 1799 | 1648 |
| django | 0.99984 | 2772 | 34 |
| home-assistant | 0.99963 | 10596 | 146 |
| poetry | 0.99925 | 317 | 12 |
| transformers | 0.99967 | 2657 | 322 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99980 | 3669 | 18 |
| warehouse | 0.99977 | 654 | 13 |
| zulip | 0.99970 | 1459 | 21 |

After:

| project | similarity index | total files | changed files |

|----------------|------------------:|------------------:|------------------:|
| cpython | 0.75804 | 1799 | 1648 |
| django | 0.99984 | 2772 | 34 |
| home-assistant | 0.99955 | 10596 | 213 |
| poetry | 0.99917 | 317 | 13 |
| transformers | 0.99967 | 2657 | 324 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99980 | 3669 | 18 |
| warehouse | 0.99976 | 654 | 14 |
| zulip | 0.99957 | 1459 | 36 |

crates/ruff_python_formatter/src/other/parameters.rs

@@ -252,6 +252,19 @@ impl FormatNodeRule<Parameters> for FormatParameters {
             let mut f = WithNodeLevel::new(NodeLevel::ParenthesizedExpression, f);
             // No parameters, format any dangling comments between `()`
             write!(f, [empty_parenthesized("(", dangling, ")")])
+        } else if num_parameters == 1 {
+            // If we have a single argument, avoid the inner group, to ensure that we insert a
+            // trailing comma if the outer group breaks.
+            let mut f = WithNodeLevel::new(NodeLevel::ParenthesizedExpression, f);
+            write!(
+                f,
+                [
+                    token("("),
+                    dangling_open_parenthesis_comments(parenthesis_dangling),
+                    soft_block_indent(&format_inner),
+                    token(")")
+                ]
+            )
         } else {
             // Intentionally avoid `parenthesized`, which groups the entire formatted contents.
             // We want parameters to be grouped alongside return types, one level up, so we