Function validate_sink_connections_exist_impl 

pub(crate) async fn validate_sink_connections_exist_impl(
    client: &Client,
    planned_project: &Project,
) -> Result<(), DatabaseValidationError>

Internal implementation of validate_sink_connections_exist.

Validates that all connections referenced by sinks exist in the database. Sinks reference connections (Kafka, Iceberg) that are not managed by mz-deploy.