tests: add chat model tests(cv: 100)

test/dashbot/models/chat_state_model_test.dart

@@ -0,0 +1,59 @@
+import 'package:apidash/dashbot/core/error/chat_failure.dart';
+import 'package:apidash/dashbot/features/chat/models/chat_message.dart';
+import 'package:apidash/dashbot/features/chat/models/chat_state.dart';
+import 'package:apidash/dashbot/core/constants/constants.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('ChatState copyWith & integrity', () {
+    test('copyWith updates provided fields only', () {
+      final msg = ChatMessage(
+        id: '1',
+        content: 'Ping',
+        role: MessageRole.user,
+        timestamp: DateTime.fromMillisecondsSinceEpoch(0),
+      );
+      const failure = ChatFailure('generic');
+      const base = ChatState();
+      final next = base.copyWith(
+        chatSessions: {
+          'r1': [msg]
+        },
+        isGenerating: true,
+        currentStreamingResponse: 'stream',
+        currentRequestId: 'r1',
+        lastError: failure,
+      );
+      expect(next.chatSessions['r1']!.single, msg);
+      expect(next.isGenerating, true);
+      expect(next.currentStreamingResponse, 'stream');
+      expect(next.currentRequestId, 'r1');
+      expect(next.lastError, failure);
+      // Original untouched
+      expect(base.chatSessions, isEmpty);
+      expect(base.isGenerating, false);
+
+      // Calling copyWith with NO arguments hits fallback (right side of ??)
+      final same = next.copyWith();
+      expect(same.chatSessions, next.chatSessions);
+      expect(same.isGenerating, next.isGenerating);
+      expect(same.currentStreamingResponse, next.currentStreamingResponse);
+      expect(same.currentRequestId, next.currentRequestId);
+      expect(same.lastError, next.lastError);
+
+      // Explicit null parameters should also fall back to existing values
+      final viaNulls = next.copyWith(
+        chatSessions: null,
+        isGenerating: null,
+        currentStreamingResponse: null,
+        currentRequestId: null,
+        lastError: null,
+      );
+      expect(viaNulls.chatSessions, next.chatSessions);
+      expect(viaNulls.isGenerating, next.isGenerating);
+      expect(viaNulls.currentStreamingResponse, next.currentStreamingResponse);
+      expect(viaNulls.currentRequestId, next.currentRequestId);
+      expect(viaNulls.lastError, next.lastError);
+    });
+  });
+}