Avoid hard line break after dangling open-parenthesis comments (#6380)

## Summary

Given:

```python
[  # comment
    first,
    second,
    third
]  # another comment
```

We were adding a hard line break as part of the formatting of `#
comment`, which led to the following formatting:

```python
[first, second, third]  # comment
  # another comment
```

Closes https://github.com/astral-sh/ruff/issues/6367.

crates/ruff_python_formatter/src/comments/format.rs

@@ -227,6 +227,57 @@ impl Format<PyFormatContext<'_>> for FormatDanglingComments<'_> {
     }
 }
 
+/// Formats the dangling comments within a parenthesized expression, for example:
+/// ```python
+/// [  # comment
+///     1,
+///     2,
+///     3,
+/// ]
+/// ```
+pub(crate) fn dangling_open_parenthesis_comments(
+    comments: &[SourceComment],
+) -> FormatDanglingOpenParenthesisComments {
+    FormatDanglingOpenParenthesisComments { comments }
+}
+
+pub(crate) struct FormatDanglingOpenParenthesisComments<'a> {
+    comments: &'a [SourceComment],
+}
+
+impl Format<PyFormatContext<'_>> for FormatDanglingOpenParenthesisComments<'_> {
+    fn fmt(&self, f: &mut Formatter<PyFormatContext>) -> FormatResult<()> {
+        let mut comments = self
+            .comments
+            .iter()
+            .filter(|comment| comment.is_unformatted());
+
+        if let Some(comment) = comments.next() {
+            debug_assert!(
+                comment.line_position().is_end_of_line(),
+                "Expected dangling comment to be at the end of the line"
+            );
+
+            write!(
+                f,
+                [line_suffix(&format_args![
+                    space(),
+                    space(),
+                    format_comment(comment)
+                ])]
+            )?;
+            comment.mark_formatted();
+
+            debug_assert!(
+                comments.next().is_none(),
+                "Expected at most one dangling comment"
+            );
+        }
+
+        Ok(())
+    }
+}
+
 /// Formats the content of the passed comment.
 ///
 /// * Adds a whitespace between `#` and the comment text except if the first character is a `#`, `:`, `'`, or `!`

crates/ruff_python_formatter/src/comments/mod.rs

@@ -95,8 +95,9 @@ use std::rc::Rc;
 use ruff_python_ast::{Mod, Ranged};
 
 pub(crate) use format::{
-    dangling_comments, dangling_node_comments, leading_alternate_branch_comments, leading_comments,
-    leading_node_comments, trailing_comments, trailing_node_comments,
+    dangling_comments, dangling_node_comments, dangling_open_parenthesis_comments,
+    leading_alternate_branch_comments, leading_comments, leading_node_comments, trailing_comments,
+    trailing_node_comments,
 };
 use ruff_formatter::{SourceCode, SourceCodeSlice};
 use ruff_python_ast::node::AnyNodeRef;