wrong datetime conversion when sending SmS

[junoyoon]

https://github.com/infobip/infobip-api-java-client/blob/f6889abeb4052e6077816f06e82fff909919fb99/src/main/java/com/infobip/JSON.java#L31C1-L32C71

this datetype is wrong for SMS.
For SMS, https://www.infobip.com/docs/api/channels/sms/outbound-sms/send-message/send-sms-messages
options > schedule > sendAt parameter require yyyy-MM-dd'T'HH:mm:ss.SSSZ
this ends with 'Z' and means UTC time.

but the DateTimeFormatter in SDK 'Z' means UTC but timezone offset (RFC 822) like +0900
This causes the imcompatibility b/w SDK and infobip api.

success

curl --request POST 'https://6zjqez.api.infobip.com/email/3/send' \
...
--form 'sendAt="2026-03-12T04:59:59.123Z"'

error

curl --request POST 'https://6zjqez.api.infobip.com/email/3/send' \
...
--form 'sendAt="2026-03-12T04:59:59.123+0000"'

To fix this,
the DateTimeFomatter in https://github.com/infobip/infobip-api-java-client/blob/f6889abeb4052e6077816f06e82fff909919fb99/src/main/java/com/infobip/JSON.java#L31C1-L32C71
should be

DateTimeFormatter f = DateTimeFormatter
        .ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSX")
        .withZone(ZoneOffset.UTC);