Updating MFA methods for user with TOTP active results in error

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

* Operating System version: macOS 15.6.1
* Firebase SDK version: 13.5.0
* Firebase Product: auth
* Node.js version: 20.19.2
* NPM version: 11.6.2

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

When attempted to change anything in the MFA configuration of a user that has TOTP active, firebase throws.

#### Steps to reproduce:

See the following example, where I attempt to enroll user with `phone` factor.

```ts
let enrolledFactors = (user.multiFactor?.enrolledFactors || []) as unknown as firebase.auth.UpdateMultiFactorInfoRequest[];

if (enrolledFactors.find((f) => f.factorId === method)) {
console.log(`User ${email} is already enrolled in MFA method: ${method}`);
return;
}

await firebase.auth().updateUser(user.uid, {
multiFactor: {
enrolledFactors: [
...enrolledFactors,
{
uid: `phone:${sanitizePhoneNumber(phoneNumber)}`,
factorId: FactorId.PHONE,
phoneNumber,
},
],
},
});
```

If the `enrolledFactors` include an entry with `factorId` `totp`, it throws:

```
FirebaseAuthError: Unsupported second factor "{"uid":"58637419-3818-4e95-b26c-7254aa11a85b","displayName":"TOTP","factorId":"totp","enrollmentTime":"Fri, 24 Oct 2025 09:34:05 GMT","totpInfo":{}}" provided.
```

It does look like the firebase-admin does not support TOTP fully, even though user was able to enroll and use the TOTP method on the client just fine - and the `user.multiFactor?.enrolledFactors` do return it (but you can't save it back to the firebase).