feat(learn): the assistant's conversation a dialog on the desktop and in expanded windows (Todo's ADR 0033, D11)

- halkyon_ui's sheetShowsAsDialog says whether showAdaptiveSheet shows
  a dialog here, for a sheet of its own that can't go through it.
- Learn's conversation, a half-height sheet dragged up, asks it: where
  sheets are dialogs it is a dialog up to 720 wide and as tall as the
  window, with no handle to drag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/test/feature/assistant/conversation_test.dart

@@ -1,6 +1,6 @@
 import 'dart:convert';
-import 'dart:typed_data';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:halkyon_app/halkyon_app.dart';
 import 'package:halkyon_app/testing.dart';
@@ -265,6 +265,30 @@ void main() {
     );
   });
 
+  testWidgets('on the desktop the conversation opens in a dialog, however '
+      "narrow the window, with no handle to drag (Todo's ADR 0033, D11)", (
+    tester,
+  ) async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.linux;
+    addTearDown(() => debugDefaultTargetPlatformOverride = null);
+    await showApp(tester);
+    assistants.answers.add(_says('Because it joins clauses.'));
+
+    await ask(tester, 'Why て-form?');
+
+    expect(find.byType(Dialog), findsOneWidget);
+    expect(find.byType(BottomSheet), findsNothing);
+    expect(find.byType(DraggableScrollableSheet), findsNothing);
+    expect(
+      find.text('Because it joins clauses.', findRichText: true),
+      findsOneWidget,
+    );
+    await tester.tap(find.byTooltip('Close'));
+    await tester.pumpAndSettle();
+    expect(find.byType(Dialog), findsNothing);
+    debugDefaultTargetPlatformOverride = null;
+  });
+
   testWidgets('an assistant not agreed to asks for agreement first, sending '
       'nothing (D40)', (tester) async {
     assistants.offer = AssistantsOffer(assistants: [_claude(agreed: false)]);