cover Vim edit-ops and extension edge branches

Lift line coverage back over the 95% floor after the Flutter 3.44.1
merge nudged it to 94.99%. Adds tests for the changeWord/no-op-edit/
empty-paste/word-motion-edge branches in vim_edit_ops, the lone-key
flush path in SequenceMatcher, and the keybindings-ui / vim extension
identity + deactivate paths.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/builtin/vim/vim_extension_test.dart

@@ -22,6 +22,12 @@ void main() {
 
   bool flag(String name) => f.services.keymap.scope[name] ?? false;
 
+  test('identifies itself', () {
+    expect(ext.id, 'builtin.vim');
+    expect(ext.title, 'Vim');
+    expect(ext.version, '0.1.0');
+  });
+
   test('inert under the default preset', () async {
     await f.services.extensions.activate('builtin.vim');
     expect(ext.modeService!.enabled, isFalse);