fix(coderd): fix template ai task check error message (#20651)

Create task was still mentioning magic prompt parameter when checking
template task validity. This change updates it to only mention validity
of `coder_ai_task` resource.

coderd/aitasks.go

@@ -92,7 +92,7 @@ func (api *API) tasksCreate(rw http.ResponseWriter, r *http.Request) {
 
 	if !templateVersion.HasAITask.Valid || !templateVersion.HasAITask.Bool {
 		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
-			Message: fmt.Sprintf(`Template does not have required parameter %q`, codersdk.AITaskPromptParameterName),
+			Message: `Template does not have a valid "coder_ai_task" resource.`,
 		})
 		return
 	}