fix(sharebymail): Fix activity rich object id type

Fixes "Object for placeholder file is invalid, value 21 for key id is not a string"
Exception spotted in tests logs.

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

apps/sharebymail/lib/Activity.php

@@ -233,12 +233,12 @@ class Activity implements IProvider {
 	/**
 	 * @param int $id
 	 * @param string $path
-	 * @return array
+	 * @return array<string,string>
 	 */
-	protected function generateFileParameter($id, $path) {
+	protected function generateFileParameter($id, $path): array {
 		return [
 			'type' => 'file',
-			'id' => $id,
+			'id' => (string)$id,
 			'name' => basename($path),
 			'path' => trim($path, '/'),
 			'link' => $this->url->linkToRouteAbsolute('files.viewcontroller.showFile', ['fileid' => $id]),