REST: Add AssertStatementSubjectExists use case step

Bug: T342889
Change-Id: I933bdcba896fba2ee88cc7d742189b101a32fa7e

repo/rest-api/src/Application/UseCases/AssertStatementSubjectExists.php

@@ -0,0 +1,26 @@
+<?php declare( strict_types=1 );
+
+namespace Wikibase\Repo\RestApi\Application\UseCases;
+
+use Wikibase\DataModel\Entity\EntityId;
+
+/**
+ * @license GPL-2.0-or-later
+ */
+class AssertStatementSubjectExists {
+
+	private GetLatestStatementSubjectRevisionMetadata $getRevisionMetadata;
+
+	public function __construct( GetLatestStatementSubjectRevisionMetadata $getRevisionMetadata ) {
+		$this->getRevisionMetadata = $getRevisionMetadata;
+	}
+
+	/**
+	 * @throws ItemRedirect if the statement subject is a redirect
+	 * @throws UseCaseError if the statement subject does not exist
+	 */
+	public function execute( EntityId $id ): void {
+		$this->getRevisionMetadata->execute( $id );
+	}
+
+}