TF-3786 Add Return-Path header when request read receipt (#3790)

lib/features/thread/domain/model/search_email.dart

@@ -33,6 +33,7 @@ class SearchEmail extends Email {
     super.bodyValues,
     super.headerUserAgent,
     super.headerMdn,
+    super.headerReturnPath,
     super.headerCalendarEvent,
     super.xPriorityHeader,
     super.importanceHeader,
@@ -84,6 +85,7 @@ class SearchEmail extends Email {
       bodyValues: email.bodyValues,
       headerUserAgent: email.headerUserAgent,
       headerMdn: email.headerMdn,
+      headerReturnPath: email.headerReturnPath,
       headerCalendarEvent: email.headerCalendarEvent,
       xPriorityHeader: email.xPriorityHeader,
       importanceHeader: email.importanceHeader,