'no transaction in use' when instantiating the SessionFactory manually and trying to persist via EntityManager

Question

I am creating the session factory by myself (not via Spring infra) - see minimal working example below. I am using Hibernate 6.4.4.

I think I need to somehow provide it an transaction manager (HibernateTransactionManager?), so it can use it then in the beginTransaction, but how?

@Test
fun insertViaEntityManagerFails() {
    val metadata = ensureSchema(MyEntity::class.java)
    val sessionFactory = metadata.buildSessionFactory()

    sessionFactory.openSession().use {
        val t = it.beginTransaction()
        val em = it.entityManagerFactory.createEntityManager()
        em.persist(MyEntity(id = 777))

        em.flush() // This throws 'no transaction is in progress' (does not matter whether I call beginTransaction() or not)
        t.commit()
    }
}

@Test
fun insertViaStateLessSessionWorks() {
    val metadata = ensureSchema(MyEntity::class.java)
    val sessionFactory = metadata.buildSessionFactory()

    sessionFactory.openStatelessSession().use {
        it.insert(MyEntity(id = 1234))
    }

    // This works and the entity is inserted correctly
}

private val dataSource = DataSourceBuilder... // Db connection here

private fun createMetadata(classes: Array<out Class<*>>): Metadata {
    val registry = StandardServiceRegistryBuilder().applySettings(
        mapOf("hibernate.connection.datasource" to dataSource)
    ).build()

    return MetadataSources(registry).addAnnotatedClasses(*classes).metadataBuilder.build()
}

private fun ensureSchema(vararg classes: Class<*>): Metadata {
    val metadata = createMetadata(batchSize, classes)

    val schemaUpdate = SchemaExport()
    schemaUpdate.execute(EnumSet.of(TargetType.DATABASE), SchemaExport.Action.BOTH, metadata)

    if (schemaUpdate.exceptions.isNotEmpty()) {
        throw Exception("Schema update failed")
    }
    return metadata
}