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/kernel/src/theme/mop_up_test.dart

@@ -6,10 +6,8 @@
 library;
 
 import 'dart:io';
-import 'dart:ui';
 
 import 'package:clide/kernel/kernel.dart';
-import 'package:clide/kernel/src/theme/contrast.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';