[isort] Handle multiple continuation lines after module docstring (I002) (#19818)

## Summary

Fixes #19815

---------

Co-authored-by: Brent Westbrook <36778786+ntBre@users.noreply.github.com>

crates/ruff_linter/resources/test/fixtures/isort/required_imports/docstring_with_multiple_continuations.py

@@ -0,0 +1,4 @@
+"""Hello, world!"""\
+\
+
+x = 1; y = 2

crates/ruff_linter/src/importer/insertion.rs

@@ -63,9 +63,9 @@ impl<'a> Insertion<'a> {
                 return Insertion::inline(" ", location.add(offset).add(TextSize::of(';')), ";");
             }
 
-            // If the first token after the docstring is a continuation character (i.e. "\"), advance
-            // an additional row to prevent inserting in the same logical line.
-            if match_continuation(locator.after(location)).is_some() {
+            // While the first token after the docstring is a continuation character (i.e. "\"), advance
+            // additional rows to prevent inserting in the same logical line.
+            while match_continuation(locator.after(location)).is_some() {
                 location = locator.full_line_end(location);
             }
 
@@ -379,6 +379,17 @@ mod tests {
             Insertion::own_line("", TextSize::from(22), "\n")
         );
 
+        let contents = r#"
+"""Hello, world!"""\
+\
+
+"#
+        .trim_start();
+        assert_eq!(
+            insert(contents)?,
+            Insertion::own_line("", TextSize::from(24), "\n")
+        );
+
         let contents = r"
 x = 1
 "

crates/ruff_linter/src/rules/isort/mod.rs

@@ -797,6 +797,7 @@ mod tests {
     #[test_case(Path::new("docstring_followed_by_continuation.py"))]
     #[test_case(Path::new("docstring_only.py"))]
     #[test_case(Path::new("docstring_with_continuation.py"))]
+    #[test_case(Path::new("docstring_with_multiple_continuations.py"))]
     #[test_case(Path::new("docstring_with_semicolon.py"))]
     #[test_case(Path::new("empty.py"))]
     #[test_case(Path::new("existing_import.py"))]
@@ -832,6 +833,7 @@ mod tests {
     #[test_case(Path::new("docstring_followed_by_continuation.py"))]
     #[test_case(Path::new("docstring_only.py"))]
     #[test_case(Path::new("docstring_with_continuation.py"))]
+    #[test_case(Path::new("docstring_with_multiple_continuations.py"))]
     #[test_case(Path::new("docstring_with_semicolon.py"))]
     #[test_case(Path::new("empty.py"))]
     #[test_case(Path::new("existing_import.py"))]

crates/ruff_linter/src/rules/isort/snapshots/ruff_linter__rules__isort__tests__required_import_docstring_with_multiple_continuations.py.snap

@@ -0,0 +1,13 @@
+---
+source: crates/ruff_linter/src/rules/isort/mod.rs
+---
+I002 [*] Missing required import: `from __future__ import annotations`
+--> docstring_with_multiple_continuations.py:1:1
+help: Insert required import: `from __future__ import annotations`
+
+ℹ Safe fix
+1 1 | """Hello, world!"""\
+2 2 | \
+3 3 | 
+  4 |+from __future__ import annotations
+4 5 | x = 1; y = 2

crates/ruff_linter/src/rules/isort/snapshots/ruff_linter__rules__isort__tests__required_import_with_alias_docstring_with_multiple_continuations.py.snap

@@ -0,0 +1,13 @@
+---
+source: crates/ruff_linter/src/rules/isort/mod.rs
+---
+I002 [*] Missing required import: `from __future__ import annotations as _annotations`
+--> docstring_with_multiple_continuations.py:1:1
+help: Insert required import: `from __future__ import annotations as _annotations`
+
+ℹ Safe fix
+1 1 | """Hello, world!"""\
+2 2 | \
+3 3 | 
+  4 |+from __future__ import annotations as _annotations
+4 5 | x = 1; y = 2