validateLegacyPhoneKeys
fun validateLegacyPhoneKeys(userId: String, serverAddress: String, forceGeneration: Boolean = false, callback: (ValidatePhoneKeysResult) -> Unit)
Ensures a P-256 key pair exists for the given user and server, generating and persisting one if needed.
Updates config.phoneKeysAlias to baseAlias + userId + serverAddress before checking keys.