fix: use Headers API in defaultArmRequest to match apim-client.ts pattern

Agent-Logs-Url: https://github.com/Azure/apiops-cli/sessions/16641d14-86ab-42af-9e18-34ecbc7c4bd1

Co-authored-by: EMaher <9244742+EMaher@users.noreply.github.com>

Author: Copilot

src/services/keyvault-checker.ts

@@ -89,13 +89,11 @@ function defaultTokenProviderFactory(): TokenProvider {
 }
 
 async function defaultArmRequest(url: string, token: string): Promise<ArmResponse> {
-  const response = await fetch(url, {
-    headers: {
-      Authorization: `Bearer ${token}`,
-      Accept: 'application/json',
-      'User-Agent': USER_AGENT,
-    },
-  });
+  const headers = new Headers();
+  headers.set('Authorization', `Bearer ${token}`);
+  headers.set('Accept', 'application/json');
+  headers.set('User-Agent', USER_AGENT);
+  const response = await fetch(url, { headers });
   return {
     status: response.status,
     json: () => response.json() as Promise<unknown>,