markdown: Don't adjust indentation when inserting with multiple cursors (#40794)

Closes #40757

## Summary

This PR addresses an issue where Zed incorrectly adjusts the indentation
of Markdown lists when inserting text using multiple cursors. Currently:

- Editing individual lines with a single cursor behaves correctly (no
unwanted indentation changes).
- Using multiple cursors, Zed automatically adjusts the indentation,
unlike VS Code, which preserves the existing formatting.

## Tasks
- [x] Implement a new test to verify correct Markdown indentation
behavior with multiple cursors.
- [x] Apply the fix to prevent Zed from adjusting indentation when
inserting text on multiple cursors.

------------------------

Release Notes:

- Fixed an issue where inserting text with multiple cursors inside a
nested Markdown list would cause it to lose its indentation.

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

crates/editor/src/editor_tests.rs

@@ -27498,6 +27498,65 @@ async fn test_paste_url_from_other_app_creates_markdown_link_over_selected_text(
     ));
 }
 
+#[gpui::test]
+async fn test_markdown_list_indent_with_multi_cursor(cx: &mut gpui::TestAppContext) {
+    init_test(cx, |_| {});
+
+    let markdown_language = languages::language("markdown", tree_sitter_md::LANGUAGE.into());
+    let mut cx = EditorTestContext::new(cx).await;
+
+    cx.update_buffer(|buffer, cx| buffer.set_language(Some(markdown_language), cx));
+
+    cx.set_state(&indoc! {"
+        - [ ] Item 1
+            - [ ] Item 1.a
+        - [ˇ] Item 2
+            - [ˇ] Item 2.a
+            - [ˇ] Item 2.b
+        "
+    });
+
+    cx.update_editor(|editor, window, cx| {
+        editor.handle_input("X", window, cx);
+    });
+
+    cx.assert_editor_state(indoc! {"
+        - [ ] Item 1
+            - [ ] Item 1.a
+        - [Xˇ] Item 2
+            - [Xˇ] Item 2.a
+            - [Xˇ] Item 2.b
+        "
+    });
+}
+
+#[gpui::test]
+async fn test_markdown_list_indent_with_newline(cx: &mut gpui::TestAppContext) {
+    init_test(cx, |_| {});
+
+    let markdown_language = languages::language("markdown", tree_sitter_md::LANGUAGE.into());
+    let mut cx = EditorTestContext::new(cx).await;
+
+    cx.update_buffer(|buffer, cx| buffer.set_language(Some(markdown_language), cx));
+
+    cx.set_state(indoc! {"
+        - [x] list item
+          - [x] sub list itemˇ
+        "
+    });
+
+    cx.update_editor(|editor, window, cx| {
+        editor.newline(&Newline, window, cx);
+    });
+
+    cx.assert_editor_state(indoc! {"
+        - [x] list item
+          - [x] sub list item
+          ˇ
+        "
+    });
+}
+
 #[gpui::test]
 async fn test_paste_url_from_zed_copy_creates_markdown_link_over_selected_text(
     cx: &mut gpui::TestAppContext,