Preserve newlines after nested compound statements (#7608)

## Summary

Given:
```python
if True:
    if True:
        pass
    else:
        pass
        # a

        # b
        # c

else:
    pass
```

We want to preserve the newline after the `# c` (before the `else`).
However, the `last_node` ends at the `pass`, and the comments are
trailing comments on the `pass`, not trailing comments on the
`last_node` (the `if`). As such, when counting the trailing newlines on
the outer `if`, we abort as soon as we see the comment (`# a`).

This PR changes the logic to skip _all_ comments (even those with
newlines between them). This is safe as we know that there are no
"leading" comments on the `else`, so there's no risk of skipping those
accidentally.

Closes https://github.com/astral-sh/ruff/issues/7602.

## Test Plan

No change in compatibility.

Before:

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

|--------------|------------------:|------------------:|------------------:|
| cpython | 0.76083 | 1789 | 1631 |
| django | 0.99983 | 2760 | 36 |
| transformers | 0.99963 | 2587 | 319 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99979 | 3496 | 22 |
| warehouse | 0.99967 | 648 | 15 |
| zulip | 0.99972 | 1437 | 21 |

After:

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

|--------------|------------------:|------------------:|------------------:|
| cpython | 0.76083 | 1789 | 1631 |
| django | 0.99983 | 2760 | 36 |
| transformers | 0.99963 | 2587 | 319 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99983 | 3496 | 18 |
| warehouse | 0.99967 | 648 | 15 |
| zulip | 0.99972 | 1437 | 21 |

crates/ruff_python_trivia/src/tokenizer.rs

@@ -88,10 +88,33 @@ pub fn lines_after(offset: TextSize, code: &str) -> u32 {
     newlines
 }
 
+/// Counts the empty lines after `offset`, ignoring any trailing trivia: end-of-line comments,
+/// own-line comments, and any intermediary newlines.
+pub fn lines_after_ignoring_trivia(offset: TextSize, code: &str) -> u32 {
+    let mut newlines = 0u32;
+    for token in SimpleTokenizer::starts_at(offset, code) {
+        match token.kind() {
+            SimpleTokenKind::Newline => {
+                newlines += 1;
+            }
+            SimpleTokenKind::Whitespace => {}
+            // If we see a comment, reset the newlines counter.
+            SimpleTokenKind::Comment => {
+                newlines = 0;
+            }
+            // As soon as we see a non-trivia token, we're done.
+            _ => {
+                break;
+            }
+        }
+    }
+    newlines
+}
+
 /// Counts the empty lines after `offset`, ignoring any trailing trivia on the same line as
 /// `offset`.
 #[allow(clippy::cast_possible_truncation)]
-pub fn lines_after_ignoring_trivia(offset: TextSize, code: &str) -> u32 {
+pub fn lines_after_ignoring_end_of_line_trivia(offset: TextSize, code: &str) -> u32 {
     // SAFETY: We don't support files greater than 4GB, so casting to u32 is safe.
     SimpleTokenizer::starts_at(offset, code)
         .skip_while(|token| token.kind != SimpleTokenKind::Newline && token.kind.is_trivia())