refactor: change accountId from optional to required in MailtrapClient class

Author: narekhovhannisyan

src/lib/MailtrapClient.ts

@@ -45,7 +45,7 @@ export default class MailtrapClient {
 
   private testInboxId?: number;
 
-  private accountId?: number;
+  private accountId: number;
 
   private bulk: boolean;