feat(todo): comments on a task (ADR 0033, D36)

Plain text with links, written, edited and deleted in the task's sheet,
counted on its row. They are sealed in `task_comment`, the task they belong
to with them, and go with their task.

Whats-New: Comments: write on a task, edit or delete what you wrote, and open its links.
Whats-New-De: Kommentare: Schreibe zu einer Aufgabe, bearbeite oder lösche es und öffne Links darin.

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/todo/halkyon_todo_flutter/test/feature/comment/comment_test.dart

@@ -0,0 +1,80 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_todo_flutter/src/core/sealed/json_payload.dart';
+import 'package:halkyon_todo_flutter/src/feature/comment/data/comment_payload.dart';
+import 'package:halkyon_todo_flutter/src/feature/comment/model/comment.dart';
+
+void main() {
+  final written = DateTime.utc(2026, 11, 6, 19, 16);
+
+  group("a comment's payload", () {
+    test('comes back as it went, edited or not', () {
+      final plain = CommentContent(
+        taskId: 't1',
+        text: 'Ask the vet',
+        writtenAt: written,
+      );
+      final edited = CommentContent(
+        taskId: 't1',
+        text: 'Ask the vet again',
+        writtenAt: written,
+        editedAt: written.add(const Duration(hours: 1)),
+      );
+
+      expect(decodeComment(encodeComment(plain)), plain);
+      expect(decodeComment(encodeComment(edited)), edited);
+    });
+
+    test('of a later schema is not read', () {
+      expect(
+        decodeComment(encodeJson(commentSchemaVersion + 1, {'taskId': 't'})),
+        isNull,
+      );
+    });
+  });
+
+  group('links in a comment', () {
+    List<String?> urls(String text) => [
+      for (final piece in linkPieces(text)) piece.url?.toString(),
+    ];
+
+    test('plain text stays one piece', () {
+      expect(linkPieces('Nothing here'), [
+        (text: 'Nothing here', url: null),
+      ]);
+    });
+
+    test('a link is found, without the sentence ending after it', () {
+      expect(linkPieces('See https://example.org/a. Then go'), [
+        (text: 'See ', url: null),
+        (
+          text: 'https://example.org/a',
+          url: Uri.parse('https://example.org/a'),
+        ),
+        (text: '. Then go', url: null),
+      ]);
+    });
+
+    test('nothing is lost between the pieces', () {
+      const text = 'a http://one.example b https://two.example/x?y=1 c';
+
+      expect(linkPieces(text).map((piece) => piece.text).join(), text);
+      expect(urls(text).nonNulls, [
+        'http://one.example',
+        'https://two.example/x?y=1',
+      ]);
+    });
+
+    test('nothing but http and https ever links', () {
+      for (final text in [
+        'javascript:alert(1)',
+        'file:///etc/passwd',
+        'intent://scan#Intent;end',
+        'tel:+491234',
+        'mailto:someone@example.org',
+        'https://',
+      ]) {
+        expect(urls(text).nonNulls, isEmpty, reason: text);
+      }
+    });
+  });
+}