Write unit test for sMimeStatus method

model/test/extensions/list_email_header_extension_test.dart

@@ -0,0 +1,48 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:jmap_dart_client/jmap/mail/email/email_header.dart';
+import 'package:model/email/email_property.dart';
+import 'package:model/extensions/list_email_header_extension.dart';
+
+void main() {
+  group('ListEmailHeaderExtension::test', () {
+    test('sMimeStatus returns correct value when header `X-SMIME-Status` is present', () {
+      final emailHeaders = {
+        EmailHeader(EmailProperty.headerSMimeStatusKey, 'Good signature')
+      };
+      expect(emailHeaders.sMimeStatus, 'Good signature');
+    });
+
+    test('sMimeStatus returns empty string when header is absent', () {
+      final emailHeaders = {
+        EmailHeader('OtherHeader', 'Value')
+      };
+      expect(emailHeaders.sMimeStatus, '');
+    });
+
+    test('sMimeStatus returns empty string for null set', () {
+      Set<EmailHeader>? emailHeaders;
+      expect(emailHeaders.sMimeStatus, '');
+    });
+
+    test('sMimeStatus trims start whitespace from the value', () {
+      final emailHeaders = {
+        EmailHeader(EmailProperty.headerSMimeStatusKey, ' Good signature')
+      };
+      expect(emailHeaders.sMimeStatus, 'Good signature');
+    });
+
+    test('sMimeStatus trims end whitespace from the value', () {
+      final emailHeaders = {
+        EmailHeader(EmailProperty.headerSMimeStatusKey, 'Good signature ')
+      };
+      expect(emailHeaders.sMimeStatus, 'Good signature');
+    });
+
+    test('sMimeStatus trims start-end whitespace from the value', () {
+      final emailHeaders = {
+        EmailHeader(EmailProperty.headerSMimeStatusKey, ' Good signature ')
+      };
+      expect(emailHeaders.sMimeStatus, 'Good signature');
+    });
+  });
+}