HttpAgent can not work when serviceAccountPathOrObject parameter is a object in admin.credential.cert()

[FTD-YI]

Problem

When calling admin.credential.cert(), and the serviceAccountPathOrObject parameter is a object, not a file path, the http proxy agent can not work.

Environment

• Operating System version: Red Hat Enterprice Linux 8.10
• Firebase SDK version: 13.4.0
• Firebase Product: message
• Node.js version: 22.11.0
• NPM version: 10.9.0

Steps to reproduce:

#### Relevant Code:

import * as admin from "firebase-admin";
import {readFilesync }from "fs";
import * as https from "https",

const serviceAccount = JSON.parse(readFilesync("./serviceAccount.json", "utf8"))
const proxyAgent = new https.Agent({
  host:"xxx",
  port: 8080,
  keepAlive: true,
  rejectUnauthorized: false,
});
const client= admin.initializeApp({
  credential:admin.credential.cert(
    {
      clientEmail: serviceAccount.client_email,
      privatekey: serviceAccount.private_key,
      projectId: serviceAccount.project_id,
    },// when serviceAccount is a object, it will raise the "app/invalid-credential" error
    // "./serviceAccount.json", when serviceAccount is a file path, can work
    proxyAgent
  ),
  httpAgent: proxyAgent, 
});

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[FTD-YI]

Raise a PR to solve this issue, #2961, please review.