Incorrect type for CreateRequest.phoneNumber

[clintonb]

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[REQUIRED] Step 2: Describe your environment

• Operating System version: _____
• Firebase SDK version: 13.4.0
• Firebase Product: auth
• Node.js version: 22.11
• NPM version: ??? (I use pnpm)

[REQUIRED] Step 3: Describe the problem

Per https://github.com/firebase/firebase-admin-node/issues/1508, phoneNumber is not nullable for create requests.

Steps to reproduce:

1. Try to create a user with phoneNumber = null.
2. Get an error from the API

[clintonb]

This is also somewhat related to https://github.com/firebase/firebase-admin-node/issues/2450.