UserNotFoundError is not raised as expected. Instead, UnexpectedResponseError is raised.

### [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__, report directly in the
[Python Firestore](https://github.com/googleapis/python-firestore) GitHub repo. Firestore
bugs reported in this repo will be closed with a reference to the Python Firestore
project.
* For general technical questions, post a question on [StackOverflow](http://stackoverflow.com/)
with the `firebase` tag.
* For general Firebase discussion, use the [firebase-talk](https://groups.google.com/forum/#!forum/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](https://firebase.google.com/support/).

### [REQUIRED] Step 2: Describe your environment

* Operating System version: 14.2.1（23C71）
* Firebase SDK version: 6.3.0
* Firebase Product: auth
* Python version:3.12.0
* Pip version: 24.0

### [REQUIRED] Step 3: Describe the problem

When calling the `generate_password_reset_link`, we got errors `Failed to generate email action link.` which is raised in `_auth_utils.UnexpectedResponseError`.
We expect UserNotFoundError to be raised if the email in request is not in authentication list.

#### Steps to reproduce:

What happened? How can we make the problem occur?
This could be a description, log/console output, etc.

Calling `generate_password_reset_link` with invalid email

#### Relevant Code:

```
def reset_password(
self, request: PostResetPasswordRequest, lang: str, client: MailjetClient
):
try:
password_link = auth.generate_password_reset_link(request.email)
except auth.UserNotFoundError:
# For security reasons, we don't want to reveal whether the email exists or not
logging.info(
f"Password reset request for non-existing email: {request.email}"
)
return None

except Exception as e:
logging.error(f"Failed to accept password reset request: {e}")
return None
```

```
def generate_email_action_link(self, action_type, email, action_code_settings=None):
"""Fetches the email action links for types

Args:
action_type: String. Valid values ['VERIFY_EMAIL', 'EMAIL_SIGNIN', 'PASSWORD_RESET']
email: Email of the user for which the action is performed
action_code_settings: ``ActionCodeSettings`` object or dict (optional). Defines whether
the link is to be handled by a mobile app and the additional state information to be
passed in the deep link, etc.
Returns:
link_url: action url to be emailed to the user

Raises:
UnexpectedResponseError: If the backend server responds with an unexpected message
FirebaseError: If an error occurs while generating the link
ValueError: If the provided arguments are invalid
"""
payload = {
'requestType': _auth_utils.validate_action_type(action_type),
'email': _auth_utils.validate_email(email),
'returnOobLink': True
}

if action_code_settings:
payload.update(encode_action_code_settings(action_code_settings))

body, http_resp = self._make_request('post', '/accounts:sendOobCode', json=payload)
if not body or not body.get('oobLink'):
raise _auth_utils.UnexpectedResponseError(
'Failed to generate email action link.', http_response=http_resp)
return body.get('oobLink')
```