test sweep: kernel commands + widgets coverage, ratchet floor to 93

Add tests for `keybindings.dart` (KeyEvent → Keybinding mapping,
parse-error edges, resolver entries view), `toolchain_paths.dart`
(the Flutter-free `ToolchainView.resolved` static view), and several
`widgets/src/` primitives: tooltip hover/overlay, palette filter +
submit, multitab controller `copyWith` + size getters, and additional
markdown branches (h3–h6 headings, tables, strikethrough, default
block fallback, record-link tap).

Unfreezes the pre-push coverage floor that was held at 90 on
2026-05-14 by mistake and ratchets to 93. Tidies eight test files
that had accumulated unused imports flagged by `unnecessary_import`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/widgets/multitab_controller_test.dart

@@ -162,5 +162,47 @@ void main() {
       c.remove('b');
       expect(calls, 4);
     });
+
+    test('length / isEmpty / isNotEmpty mirror the entries list', () {
+      final c = MultitabController<String>();
+      expect(c.length, 0);
+      expect(c.isEmpty, isTrue);
+      expect(c.isNotEmpty, isFalse);
+
+      c.add(entry('a'));
+      expect(c.length, 1);
+      expect(c.isEmpty, isFalse);
+      expect(c.isNotEmpty, isTrue);
+    });
+  });
+
+  group('MultitabEntry.copyWith', () {
+    test('overrides each field independently and keeps id', () {
+      const base = MultitabEntry<int>(id: 'x', title: 't', payload: 1);
+
+      final renamed = base.copyWith(title: 'T');
+      expect(renamed.id, 'x');
+      expect(renamed.title, 'T');
+      expect(renamed.payload, 1);
+      expect(renamed.closeable, isTrue);
+      expect(renamed.reorderable, isTrue);
+
+      final repayloaded = base.copyWith(payload: 99);
+      expect(repayloaded.payload, 99);
+
+      final pinned = base.copyWith(closeable: false, reorderable: false);
+      expect(pinned.closeable, isFalse);
+      expect(pinned.reorderable, isFalse);
+    });
+
+    test('omitting all overrides yields an equivalent entry', () {
+      const base = MultitabEntry<String>(id: 'a', title: 'A', payload: 'p');
+      final clone = base.copyWith();
+      expect(clone.id, base.id);
+      expect(clone.title, base.title);
+      expect(clone.payload, base.payload);
+      expect(clone.closeable, base.closeable);
+      expect(clone.reorderable, base.reorderable);
+    });
   });
 }