CreateOptOutContactLog

E.164-validate contact_number / sender_whatsapp_number on input.

Shared by the create and update serializers so an invalid number is rejected
with a 400 on POST and PATCH/PUT -- otherwise the model's normalize_number()
silently digit-strips junk (e.g. "helw" -> "") on save instead of erroring.

{
    "contact_number": "string",
    "sender_whatsapp_number": "string",
    "created_by": "string",
    "reason": "string"
}