Incorrect type for CreateRequest.phoneNumber

[clintonb]

[READ] Step 1: Are you in the right place?

• For issues related to the code in this repository file a Github issue.
• If the issue pertains to Cloud Firestore, read the instructions in the "Firestore issue"
  template.
• For general technical questions, post a question on StackOverflow
  with the firebase tag.
• For general Firebase discussion, use the firebase-talk
  google group.
• For help troubleshooting your application that does not fall under one
  of the above categories, reach out to the personalized
  Firebase support channel.

[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 #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 #2450.