scale: do not allow scaling of jobs with type system.

.changelog/16969.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+scale: Do not allow scale requests for jobs of type system
+```

nomad/job_endpoint.go

@@ -1054,9 +1054,15 @@ func (j *Job) Scale(args *structs.JobScaleRequest, reply *structs.JobRegisterRes
 		return err
 	}
 
+	// Perform validation on the job to ensure we have something that can
+	// actually be scaled. This logic can only exist here, as we need access
+	// to the job object.
 	if job == nil {
 		return structs.NewErrRPCCoded(404, fmt.Sprintf("job %q not found", args.JobID))
 	}
+	if job.Type == structs.JobTypeSystem {
+		return structs.NewErrRPCCoded(http.StatusBadRequest, `cannot scale jobs of type "system"`)
+	}
 
 	// Since job is going to be mutated we must copy it since state store methods
 	// return a shared pointer.

nomad/job_endpoint_test.go

@@ -7840,6 +7840,34 @@ func TestJobEndpoint_Scale_Priority(t *testing.T) {
 	requireAssertion.NotZero(eval.ModifyTime)
 }
 
+func TestJobEndpoint_Scale_SystemJob(t *testing.T) {
+	ci.Parallel(t)
+
+	testServer, testServerCleanup := TestServer(t, nil)
+	defer testServerCleanup()
+	codec := rpcClient(t, testServer)
+	testutil.WaitForLeader(t, testServer.RPC)
+	state := testServer.fsm.State()
+
+	mockSystemJob := mock.SystemJob()
+	must.NoError(t, state.UpsertJob(structs.MsgTypeTestSetup, 10, nil, mockSystemJob))
+
+	scaleReq := &structs.JobScaleRequest{
+		JobID: mockSystemJob.ID,
+		Target: map[string]string{
+			structs.ScalingTargetGroup: mockSystemJob.TaskGroups[0].Name,
+		},
+		Count: pointer.Of(int64(13)),
+		WriteRequest: structs.WriteRequest{
+			Region:    DefaultRegion,
+			Namespace: mockSystemJob.Namespace,
+		},
+	}
+	var resp structs.JobRegisterResponse
+	must.ErrorContains(t, msgpackrpc.CallWithCodec(codec, "Job.Scale", scaleReq, &resp),
+		`400,cannot scale jobs of type "system"`)
+}
+
 func TestJobEndpoint_InvalidCount(t *testing.T) {
 	ci.Parallel(t)
 	require := require.New(t)