Fix mediator

src/Managing.Api/Controllers/DataController.cs

@@ -702,7 +702,7 @@ public class DataController : ControllerBase
     /// </summary>
     /// <returns>An array of online agent names</returns>
     [HttpGet("GetOnlineAgent")]
-    public async Task<ActionResult<List<string>>> GetOnlineAgent()
+    public async Task<ActionResult<IEnumerable<string>>> GetOnlineAgent()
     {
         const string cacheKey = "OnlineAgentNames";