fix(apiserver): answer the controller's queue rejections as client errors (bug-1960)

Since the controller rejects submissions to closed or unknown queues
(E_XCE_QUEUE_CLOSED, E_XCE_BAD_QUEUE), the apiserver answered them as a 500
"xcecli_jobsubmit failed: -10016". SubmitJob now maps them to
backend.ErrQueueClosed / ErrQueueInvalid (numbers checked against errnox.h
at compile time), and the submit and restart handlers answer 400 naming
the queue, as the apiserver's own pre-check does. This covers a queue
closed between that check and the controller's.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/frontend/apiserver/controllers/jobs_handler_test.go

@@ -1577,3 +1577,29 @@ func TestJobHandler_SubmitRejectsNonObjectExtendData(t *testing.T) {
 		}
 	}
 }
+
+// bug-1960: a submission the controller rejects for its queue (closed after
+// the apiserver's own check, or unknown) is a client error naming the queue,
+// not a 500 with a raw error number.
+func TestJobHandler_SubmitQueueRejectedByController(t *testing.T) {
+	for _, tc := range []struct {
+		err  error
+		want string
+	}{
+		{backend.ErrQueueClosed, "queue is closed"},
+		{backend.ErrQueueInvalid, "invalid queue"},
+	} {
+		f := newHandlerFixture(t)
+		f.seedQueue("normal")
+		f.submitErr = tc.err
+		w := f.submit(map[string]interface{}{"command": "sleep 1", "queue_name": "normal"})
+		if w.Code != http.StatusBadRequest {
+			t.Errorf("%v: status = %d, want 400; body: %s", tc.err, w.Code, w.Body.String())
+			continue
+		}
+		msg, _ := decodeHandlerResponse(t, w).Msg.(string)
+		if !strings.Contains(msg, tc.want) || !strings.Contains(msg, "normal") {
+			t.Errorf("%v: msg = %q, want it to name the queue and say %q", tc.err, msg, tc.want)
+		}
+	}
+}