fix(apiserver): check the default queue exists and is open on submit (bug-1960)

Cause: preCheckJobRequest (controllers/jobs.go) ran the reserved-name,
existence and closed checks only when a queue name was given; an empty
name was set to "normal" and skipped them, so jobs were accepted into a
closed default queue.

Fix: default the name first, then run the same checks for every
submission. Two existing tests now seed the default queue. (The controller
side of bug-1960 is fixed separately.)

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

src/frontend/apiserver/controllers/jobs_handler_test.go

@@ -413,6 +413,7 @@ func TestJobHandler_SmokeSubmitOrdinaryJob(t *testing.T) {
 // as a 500 and nothing is recorded.
 func TestJobHandler_SmokeSubmitErrorIs500(t *testing.T) {
 	f := newHandlerFixture(t)
+	f.seedQueue("normal") // the default queue must exist (bug-1960)
 	f.submitErr = errors.New("backend down")
 
 	w := f.submit(map[string]interface{}{"job_name": "smoke", "command": "sleep 1"})