test(coverage): lift os/code-block/deeplink for gate headroom

The three worst-covered files were genuinely untested, not edge cases:
os.dart 27%→~85% (inject the process runner so openURL/reveal don't spawn a
real browser), clide_code_block 39%→~90% (expose the byte→char span mapper
as a top-level fn + render tests), deeplink 29%→~75% (the confirm-opens and
not-activated paths). Buys buffer above the 95% floor so a feature batch
doesn't immediately trip the gate. 95.01% → 95.22%.

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

test/builtin/deeplink/extension_test.dart

@@ -5,6 +5,7 @@ library;
 import 'dart:async';
 
 import 'package:clide/builtin/deeplink/deeplink.dart';
+import 'package:clide/extension/extension.dart' show CommandContribution;
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -53,4 +54,25 @@ void main() {
     expect(f.services.dialog.isOpen, isFalse);
     expect(r.data['status'], 'rejected');
   });
+
+  testWidgets('confirming an allowlisted link opens the file (T-56)', (tester) async {
+    await tester.pumpWidget(harness(f, const SizedBox()));
+    await tester.pump();
+
+    final future = f.services.commands.execute('deeplink.invoke', args: ['clide://open?path=/x.dart&line=5']);
+    await tester.pump();
+    expect(f.services.dialog.isOpen, isTrue);
+    f.services.dialog.dismiss(true); // confirm → the handler runs editor.open
+    await tester.pumpAndSettle();
+
+    final r = await future;
+    expect(r.data['status'], 'opened');
+    expect(r.data['path'], '/x.dart');
+  });
+
+  test('reports not-activated when invoked before activation', () async {
+    final ext = DeepLinkExtension(); // never activated → no context
+    final run = (ext.contributions.single as CommandContribution).run;
+    expect((await run(['clide://open?path=/x'])).data['status'], 'not-activated');
+  });
 }