C2S: New Add/Remove and Actor creation tests

Creating Actors via C2S doesn't make sense, thus it should fail.
Tests creating Actors with type: Application/Person/Service.

All Create Activities for new Actors currently fail with
`validator not set` in the pipeline.

test/pleroma/web/activity_pub/activity_pub_controller_test.exs

@@ -1706,6 +1706,109 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       assert note_object == Object.normalize(note_activity, fetch: false)
     end
 
+    test "it rejects Add to other user's collection", %{conn: conn} do
+      user = insert(:user)
+      target_user = insert(:user)
+
+      {:ok, activity} = CommonAPI.post(user, %{status: "Post"})
+      object = Object.normalize(activity, fetch: false)
+      object_id = object.data["id"]
+
+      data = %{
+        type: "Add",
+        target: "#{Pleroma.Web.Endpoint.url()}/users/#{target_user.nickname}/collections/featured",
+        object: object_id
+      }
+
+      conn =
+        conn
+        |> assign(:user, user)
+        |> put_req_header("content-type", "application/activity+json")
+        |> post("/users/#{user.nickname}/outbox", data)
+
+      assert json_response(conn, 400)
+    end
+
+    test "it rejects Remove to other user's collection", %{conn: conn} do
+      user = insert(:user)
+      target_user = insert(:user)
+
+      {:ok, activity} = CommonAPI.post(user, %{status: "Post"})
+      object = Object.normalize(activity, fetch: false)
+      object_id = object.data["id"]
+
+      data = %{
+        type: "Remove",
+        target: "#{Pleroma.Web.Endpoint.url()}/users/#{target_user.nickname}/collections/featured",
+        object: object_id
+      }
+
+      conn =
+        conn
+        |> assign(:user, user)
+        |> put_req_header("content-type", "application/activity+json")
+        |> post("/users/#{user.nickname}/outbox", data)
+
+      assert json_response(conn, 400)
+    end
+
+    test "it rejects creating Actors of type Application", %{conn: conn} do
+      user = insert(:user, local: true)
+
+      data = %{
+        type: "Create",
+        object: %{
+          type: "Application"
+        }
+      }
+
+      conn =
+        conn
+        |> assign(:user, user)
+        |> put_req_header("content-type", "application/json")
+        |> post("/users/#{user.nickname}/outbox", data)
+
+      assert json_response(conn, 400)
+    end
+
+    test "it rejects creating Actors of type Person", %{conn: conn} do
+      user = insert(:user, local: true)
+
+      data = %{
+        type: "Create",
+        object: %{
+          type: "Person"
+        }
+      }
+
+      conn =
+        conn
+        |> assign(:user, user)
+        |> put_req_header("content-type", "application/json")
+        |> post("/users/#{user.nickname}/outbox", data)
+
+      assert json_response(conn, 400)
+    end
+
+    test "it rejects creating Actors of type Service", %{conn: conn} do
+      user = insert(:user, local: true)
+
+      data = %{
+        type: "Create",
+        object: %{
+          type: "Service"
+        }
+      }
+
+      conn =
+        conn
+        |> assign(:user, user)
+        |> put_req_header("content-type", "application/json")
+        |> post("/users/#{user.nickname}/outbox", data)
+
+      assert json_response(conn, 400)
+    end
+
     test "it rejects like activity to object invisible to actor", %{conn: conn} do
       user = insert(:user)
       stranger = insert(:user, local: true)