Fix bad unicode calculations in do_completion (#28259)

Co-authored-by: João Marcos <marcospb19@hotmail.com>

Release Notes:

- Fixed a panic with completions around non-ASCII code

---------

Co-authored-by: João Marcos <marcospb19@hotmail.com>

crates/vim/src/normal/repeat.rs

@@ -482,6 +482,62 @@ mod test {
         );
     }
 
+    #[gpui::test]
+    async fn test_repeat_completion_unicode_bug(cx: &mut gpui::TestAppContext) {
+        VimTestContext::init(cx);
+        let cx = EditorLspTestContext::new_rust(
+            lsp::ServerCapabilities {
+                completion_provider: Some(lsp::CompletionOptions {
+                    trigger_characters: Some(vec![".".to_string(), ":".to_string()]),
+                    resolve_provider: Some(true),
+                    ..Default::default()
+                }),
+                ..Default::default()
+            },
+            cx,
+        )
+        .await;
+        let mut cx = VimTestContext::new_with_lsp(cx, true);
+
+        cx.set_state(
+            indoc! {"
+                ĩлˇк
+                ĩлк
+            "},
+            Mode::Normal,
+        );
+
+        let mut request = cx.set_request_handler::<lsp::request::Completion, _, _>(
+            move |_, params, _| async move {
+                let position = params.text_document_position.position;
+                let mut to_the_left = position;
+                to_the_left.character -= 2;
+                Ok(Some(lsp::CompletionResponse::Array(vec![
+                    lsp::CompletionItem {
+                        label: "oops".to_string(),
+                        text_edit: Some(lsp::CompletionTextEdit::Edit(lsp::TextEdit {
+                            range: lsp::Range::new(to_the_left, position),
+                            new_text: "к!".to_string(),
+                        })),
+                        ..Default::default()
+                    },
+                ])))
+            },
+        );
+        cx.simulate_keystrokes("i .");
+        request.next().await;
+        cx.condition(|editor, _| editor.context_menu_visible())
+            .await;
+        cx.simulate_keystrokes("enter escape");
+        cx.assert_state(
+            indoc! {"
+                ĩкˇ!к
+                ĩлк
+            "},
+            Mode::Normal,
+        );
+    }
+
     #[gpui::test]
     async fn test_repeat_visual(cx: &mut gpui::TestAppContext) {
         let mut cx = NeovimBackedTestContext::new(cx).await;