Add missing getAuthHeaders method to ClientContextStub

Fix TypeScript compilation error by implementing getAuthHeaders
method required by IClientContext interface.

Author: samikshya-db

tests/unit/.stubs/ClientContextStub.ts

@@ -48,4 +48,8 @@ export default class ClientContextStub implements IClientContext {
   public async getDriver(): Promise<IDriver> {
     return this.driver;
   }
+
+  public async getAuthHeaders(): Promise<HeadersInit> {
+    return {};
+  }
 }