Insert necessary blank line between class and leading comments (#8224)

## Summary

Given:

```python
# comment

class A:
    def foo(self):
        pass
```

We need to insert an additional newline between `# comment` and `class
A`. We were missing this handling for the case in which `# comment` is a
leading comment on `class A`, as opposed to a trailing comment of some
preceding statement.

In practice, I think this only applies to the specific case in which a
class or function is the first statement in a module, and there's a
single empty line between a leading comment and that class or function.
If there are no empty lines, then the comment "sticks" to the
definition; if there are two or more, then `leading_comments` will
truncate appropriately. If the class or function is nested, then we only
need one empty line anyway.

Closes https://github.com/astral-sh/ruff/issues/8215.

## Test Plan

No change in similarity.

Before:

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

|----------------|------------------:|------------------:|------------------:|
| cpython | 0.75803 | 1799 | 1647 |
| django | 0.99983 | 2772 | 34 |
| home-assistant | 0.99953 | 10596 | 186 |
| poetry | 0.99891 | 317 | 17 |
| transformers | 0.99966 | 2657 | 330 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99978 | 3669 | 20 |
| warehouse | 0.99977 | 654 | 13 |
| zulip | 0.99970 | 1459 | 22 |

After:

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

|----------------|------------------:|------------------:|------------------:|
| cpython | 0.75803 | 1799 | 1648 |
| django | 0.99983 | 2772 | 34 |
| home-assistant | 0.99953 | 10596 | 186 |
| poetry | 0.99891 | 317 | 17 |
| transformers | 0.99966 | 2657 | 330 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99978 | 3669 | 20 |
| warehouse | 0.99977 | 654 | 13 |
| zulip | 0.99970 | 1459 | 22 |

crates/ruff_python_formatter/src/comments/format.rs

@@ -507,13 +507,12 @@ fn strip_comment_prefix(comment_text: &str) -> FormatResult<&str> {
 ///
 /// For example, given:
 /// ```python
-/// def func():
+/// class Class:
 ///     ...
 /// # comment
 /// ```
 ///
 /// This builder will insert two empty lines before the comment.
-/// ```
 pub(crate) fn empty_lines_before_trailing_comments<'a>(
     f: &PyFormatter,
     comments: &'a [SourceComment],
@@ -555,3 +554,69 @@ impl Format<PyFormatContext<'_>> for FormatEmptyLinesBeforeTrailingComments<'_>
         Ok(())
     }
 }
+
+/// Format the empty lines between a node and its leading comments.
+///
+/// For example, given:
+/// ```python
+/// # comment
+///
+/// class Class:
+///     ...
+/// ```
+///
+/// While `leading_comments` will preserve the existing empty line, this builder will insert an
+/// additional empty line before the comment.
+pub(crate) fn empty_lines_after_leading_comments<'a>(
+    f: &PyFormatter,
+    comments: &'a [SourceComment],
+) -> FormatEmptyLinesAfterLeadingComments<'a> {
+    // Black has different rules for stub vs. non-stub and top level vs. indented
+    let empty_lines = match (f.options().source_type(), f.context().node_level()) {
+        (PySourceType::Stub, NodeLevel::TopLevel) => 1,
+        (PySourceType::Stub, _) => 0,
+        (_, NodeLevel::TopLevel) => 2,
+        (_, _) => 1,
+    };
+
+    FormatEmptyLinesAfterLeadingComments {
+        comments,
+        empty_lines,
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+pub(crate) struct FormatEmptyLinesAfterLeadingComments<'a> {
+    /// The leading comments of the node.
+    comments: &'a [SourceComment],
+    /// The expected number of empty lines after the leading comments.
+    empty_lines: u32,
+}
+
+impl Format<PyFormatContext<'_>> for FormatEmptyLinesAfterLeadingComments<'_> {
+    fn fmt(&self, f: &mut Formatter<PyFormatContext>) -> FormatResult<()> {
+        if let Some(comment) = self
+            .comments
+            .iter()
+            .rev()
+            .find(|comment| comment.line_position().is_own_line())
+        {
+            let actual = lines_after(comment.end(), f.context().source()).saturating_sub(1);
+            // If there are no empty lines, keep the comment tight to the node.
+            if actual == 0 {
+                return Ok(());
+            }
+
+            // If there are more than enough empty lines already, `leading_comments` will
+            // trim them as necessary.
+            if actual >= self.empty_lines {
+                return Ok(());
+            }
+
+            for _ in actual..self.empty_lines {
+                write!(f, [empty_line()])?;
+            }
+        }
+        Ok(())
+    }
+}