tests: add chat model tests(cv: 100)

test/dashbot/models/chat_attachment_model_test.dart

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+import 'dart:typed_data';
+
+import 'package:apidash/dashbot/core/model/chat_attachment.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('ChatAttachment', () {
+    test('should create model with provided values', () {
+      final data = Uint8List.fromList(List<int>.generate(16, (i) => i));
+      final attachment = ChatAttachment(
+        id: 'att-1',
+        name: 'sample.txt',
+        mimeType: 'text/plain',
+        sizeBytes: data.length,
+        data: data,
+        createdAt: DateTime.utc(2024, 1, 2, 3, 4, 5),
+      );
+
+      expect(attachment.id, 'att-1');
+      expect(attachment.name, 'sample.txt');
+      expect(attachment.mimeType, 'text/plain');
+      expect(attachment.sizeBytes, 16);
+      expect(attachment.data, data);
+      expect(attachment.createdAt, DateTime.utc(2024, 1, 2, 3, 4, 5));
+    });
+
+    test('immutability (fields are final)', () {
+      final data = Uint8List.fromList([1, 2, 3]);
+      final createdAt = DateTime.now();
+      final attachment = ChatAttachment(
+        id: 'id',
+        name: 'a.bin',
+        mimeType: 'application/octet-stream',
+        sizeBytes: 3,
+        data: data,
+        createdAt: createdAt,
+      );
+
+      // Ensuring references are the same (no defensive copy inside constructor)
+      expect(identical(attachment.data, data), true);
+
+      // This test only executes field access to ensure coverage; no setters exist.
+      expect(() => (attachment.id), returnsNormally);
+    });
+  });
+}