Search 6 groundwork - Test incorrect multiple uses of DSL container contexts

Description

See org.hibernate.search.v6poc.integrationtest.backend.elasticsearch.ExtensionIT: some of the TODOs are wrong,
since in those cases there's no problem with using the contexts twice (it would just add multiple should() clauses),
but some are right. This code for instance has a problem:

SearchPredicate predicate1 = searchTarget.predicate().withExtensionOptional(
				ElasticsearchExtension.get(),
				c -> {
					c.fromJsonString( "{'match': {'string': 'text 1'}}" );
					c.fromJsonString( "{'match': {'string': 'text 2'}}" ); // What to do with this???
			  }
		);

We should test this case and other, similar cases and check an exception is thrown.