Fix blank-line docstring rules for module-level docstrings (#9878)

## Summary

Given:

```python
"""Make a summary line.

Note:
----
  Per the code comment the next two lines are blank. "// The first blank line is the line containing the closing
      triple quotes, so we need at least two."

"""
```

It turns out we excluded the line ending in `"""`, because it's empty
(unlike for functions, where it consists of the indent). This PR changes
the `following_lines` iterator to always include the trailing newline,
which gives us correct and consistent handling between function and
module-level docstrings.

Closes https://github.com/astral-sh/ruff/issues/9877.

crates/ruff_linter/src/docstrings/sections.rs

@@ -5,7 +5,7 @@ use ruff_python_ast::docstrings::{leading_space, leading_words};
 use ruff_text_size::{Ranged, TextLen, TextRange, TextSize};
 use strum_macros::EnumIter;
 
-use ruff_source_file::{Line, UniversalNewlineIterator, UniversalNewlines};
+use ruff_source_file::{Line, NewlineWithTrailingNewline, UniversalNewlines};
 
 use crate::docstrings::styles::SectionStyle;
 use crate::docstrings::{Docstring, DocstringBody};
@@ -356,13 +356,16 @@ impl<'a> SectionContext<'a> {
     pub(crate) fn previous_line(&self) -> Option<&'a str> {
         let previous =
             &self.docstring_body.as_str()[TextRange::up_to(self.range_relative().start())];
-        previous.universal_newlines().last().map(|l| l.as_str())
+        previous
+            .universal_newlines()
+            .last()
+            .map(|line| line.as_str())
     }
 
     /// Returns the lines belonging to this section after the summary line.
-    pub(crate) fn following_lines(&self) -> UniversalNewlineIterator<'a> {
+    pub(crate) fn following_lines(&self) -> NewlineWithTrailingNewline<'a> {
         let lines = self.following_lines_str();
-        UniversalNewlineIterator::with_offset(lines, self.offset() + self.data.summary_full_end)
+        NewlineWithTrailingNewline::with_offset(lines, self.offset() + self.data.summary_full_end)
     }
 
     fn following_lines_str(&self) -> &'a str {

crates/ruff_linter/src/rules/pydocstyle/rules/sections.rs

@@ -1634,12 +1634,13 @@ fn common_section(
     let line_end = checker.stylist().line_ending().as_str();
 
     if let Some(next) = next {
-        if context
-            .following_lines()
-            .last()
-            .map_or(true, |line| !line.trim().is_empty())
-        {
-            if checker.enabled(Rule::NoBlankLineAfterSection) {
+        if checker.enabled(Rule::NoBlankLineAfterSection) {
+            let num_blank_lines = context
+                .following_lines()
+                .rev()
+                .take_while(|line| line.trim().is_empty())
+                .count();
+            if num_blank_lines < 2 {
                 let mut diagnostic = Diagnostic::new(
                     NoBlankLineAfterSection {
                         name: context.section_name().to_string(),
@@ -1657,13 +1658,13 @@ fn common_section(
     } else {
         // The first blank line is the line containing the closing triple quotes, so we need at
         // least two.
-        let num_blank_lines = context
-            .following_lines()
-            .rev()
-            .take_while(|line| line.trim().is_empty())
-            .count();
-        if num_blank_lines < 2 {
-            if checker.enabled(Rule::BlankLineAfterLastSection) {
+        if checker.enabled(Rule::BlankLineAfterLastSection) {
+            let num_blank_lines = context
+                .following_lines()
+                .rev()
+                .take_while(|line| line.trim().is_empty())
+                .count();
+            if num_blank_lines < 2 {
                 let mut diagnostic = Diagnostic::new(
                     BlankLineAfterLastSection {
                         name: context.section_name().to_string(),

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__D413_D413.py.snap

@@ -21,10 +21,9 @@ D413.py:1:1: D413 [*] Missing blank line after last section ("Returns")
 7  7  | Returns:
 8  8  |     the value
    9  |+
-   10 |+
-9  11 | """
-10 12 | 
-11 13 | 
+9  10 | """
+10 11 | 
+11 12 | 
 
 D413.py:13:5: D413 [*] Missing blank line after last section ("Returns")
    |

crates/ruff_source_file/src/newlines.rs

@@ -184,11 +184,18 @@ impl<'a> Iterator for NewlineWithTrailingNewline<'a> {
     type Item = Line<'a>;
 
     #[inline]
-    fn next(&mut self) -> Option<Line<'a>> {
+    fn next(&mut self) -> Option<Self::Item> {
         self.underlying.next().or_else(|| self.trailing.take())
     }
 }
 
+impl DoubleEndedIterator for NewlineWithTrailingNewline<'_> {
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.trailing.take().or_else(|| self.underlying.next_back())
+    }
+}
+
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub struct Line<'a> {
     text: &'a str,