TF-4193 Refactor ThreadDetail: extract getLabelsForFirstEmail() to clarify label resolution logic

lib/features/thread_detail/presentation/extension/get_thread_details_email_views.dart

@@ -46,9 +46,10 @@ extension GetThreadDetailEmailViews on ThreadDetailController {
       final isFirstEmailInThreadDetail = indexOfEmailId == 0;
 
       if (presentationEmail.emailInThreadStatus == EmailInThreadStatus.collapsed) {
-        final emailLabels = labels?.isNotEmpty == true && isFirstEmailInThreadDetail
+        final emailLabels = _getLabelsForFirstEmail(
-            ? emailIdsPresentation.values.last?.getLabelList(labels!)
+          isFirstEmailInThreadDetail: isFirstEmailInThreadDetail,
-            : null;
+          availableLabels: labels,
+        );
 
         return ThreadDetailCollapsedEmail(
           presentationEmail: presentationEmail.copyWith(
@@ -109,4 +110,18 @@ extension GetThreadDetailEmailViews on ThreadDetailController {
       );
     }).toList();
   }
+
+  List<Label>? _getLabelsForFirstEmail({
+    required bool isFirstEmailInThreadDetail,
+    required List<Label>? availableLabels,
+  }) {
+    if (!isFirstEmailInThreadDetail ||
+        availableLabels == null ||
+        availableLabels.isEmpty) {
+      return null;
+    }
+
+    final lastEmail = emailIdsPresentation.values.last;
+    return lastEmail?.getLabelList(availableLabels);
+  }
 }