`get_user_by_email` can silently return wrong result when "allow multiple accounts with the same email address" is set

### Describe your environment

* Operating System version: macOS 15.2
* Firebase SDK version: `6.6.0`
* Firebase Product: `auth`
* Python version: `3.12`
* Pip version: `uv 0.5.11`

### Describe the problem

I'm worried that the `get_user_by_email` function misleadingly silently skips some results.

It's typed as returning a single user record given an email address:

https://github.com/firebase/firebase-admin-python/blob/8ba819a4175e758576f1a7cccc131c1b66d6417a/firebase_admin/_auth_client.py#L179-L194

However, it is possible to configure Firebase to allow multiple accounts with the same email address: https://support.google.com/firebase/answer/9134820

It looks like the implementation takes just the first user record if more than one matches the provided email:

https://github.com/firebase/firebase-admin-python/blob/8ba819a4175e758576f1a7cccc131c1b66d6417a/firebase_admin/_user_mgt.py#L583-L602

I'd suggest replacing it with a `get_users_by_email` that returns a collections of records, to avoid misleading programmers unfamiliar with the "allow multiple accounts with the same email address" option.