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/kernel/src/keymap/sequence_matcher_test.dart

@@ -171,5 +171,13 @@ void main() {
     test('flush with nothing buffered is a no-op', () {
       expect(matcher.flush().outcome, SeqOutcome.pending);
     });
+
+    test('flush of a lone prefix key (no exact) passes it through', () {
+      // `g` is a pure prefix of `g g` with no single-`g` binding.
+      expect(feed('g').outcome, SeqOutcome.pending);
+      final r = matcher.flush();
+      expect(r.outcome, SeqOutcome.unmatched);
+      expect(r.passKey, KeyChord.parse('g'));
+    });
   });
 }