Send or validate phone OTP
This single endpoint handles both sending and validating the phone OTP depending on which fields you provide.
**Send OTP** (first call): Provide `phoneNumber` and optionally `smsProviders`. Save the `phoneNumberOtpRequestId` from the response.
**Validate OTP** (second call): Provide `phoneNumber`, `phoneNumberOtpRequestId`, and `phoneNumberOtp`.

