Provision CTWA dataset

Creates (or fetches, if one already exists) the Meta dataset that Click-to-WhatsApp ad events are reported against via the Conversions API, and persists its ID on the account as metadata.metaCapiDatasetId.

The call is GET-first idempotent — a WABA can only own one CTWA dataset, so a second call after a successful provision is a safe no-op that returns the same ID with created: false.

Requires the connected WhatsApp account's token to carry the whatsapp_business_manage_events permission. If the permission is missing the endpoint returns 422 with a message asking the user to reconnect the account.