vim: Continue multiline comments with o (#63751)

# Objective

In Vim mode, pressing `o` to open a line inside a documentation-style
block comment preserves indentation but drops the `* ` prefix.

The behavior for pressing Return is already correct as it continues this
prefix in the next line.

This change makes both actions behave consistently.

Fixes #55049

## Solution

Make Vim’s `o` and `O` commands copy documentation-comment prefixes such
as `* ` onto the new line. This happens only when the current line is
parsed as a comment.

Also added regression coverage for this and folded it into the existing
`test_o_comment` test.

## Testing

- `CC=clang CXX=clang++ cargo test -p vim test_o_comment -- --nocapture`
- `CC=clang CXX=clang++ cargo test -p vim`
- `CC=clang CXX=clang++ cargo test -p multi_buffer`
- `cargo fmt --all -- --check`
- `CC=clang CXX=clang++ ./script/clippy -p vim -p multi_buffer`
- Tested on Linux aarch64.

## Self-Review Checklist:

- [x] I reviewed the diff for quality, security, and reliability
- [x] Unsafe blocks, if any, have justifying comments
- [x] The content adheres to the Zed UI standards
- [x] Tests cover the changed behavior
- [x] Performance impact is acceptable

---

Release Notes:

- Fixed Vim `o` comment continuation in C-style multiline comments.

crates/multi_buffer/src/multi_buffer.rs

@@ -4532,8 +4532,15 @@ impl MultiBufferSnapshot {
         if self.language_settings(cx).extend_comment_on_newline
             && let Some(language_scope) = self.language_scope_at(Point::new(row.0, 0))
         {
-            let delimiters = language_scope.line_comment_prefixes();
-            for delimiter in delimiters {
+            let documentation_delimiter = language_scope
+                .documentation_comment()
+                .filter(|_| language_scope.override_name() == Some("comment"))
+                .map(|comment| &comment.prefix);
+            for delimiter in language_scope
+                .line_comment_prefixes()
+                .iter()
+                .chain(documentation_delimiter)
+            {
                 if *self
                     .chars_at(Point::new(row.0, indent.len() as u32))
                     .take(delimiter.chars().count())

crates/vim/src/normal.rs

@@ -2174,6 +2174,10 @@ mod test {
         cx.shared_state().await.assert_eq("// hello\n// ˇ\n");
         cx.simulate_shared_keystrokes("x escape shift-o").await;
         cx.shared_state().await.assert_eq("// hello\n// ˇ\n// x\n");
+
+        cx.set_state("/*\n * helloˇ\n */\n", Mode::Normal);
+        cx.simulate_keystrokes("o");
+        cx.assert_state("/*\n * hello\n * ˇ\n */\n", Mode::Insert);
     }
 
     #[gpui::test]