merge-fix: port our scheduler/worker/apiserver fixes onto scheduler-framework merge

After merging origin/scheduler-framework (scheduler-core), re-apply and
reconcile our fixes that Rongsong's branch lacked or implemented differently:

- handler.c: keep the code<0 dispatch-fail check (bug-1512: a code-0
  success ack must NOT re-PEND, or dispatched jobs spawn duplicates).
  Use sched_job_state_change (Rongsong's enqueue mechanism) instead of
  sched_requeue_pend_job so the merged test framework (pop_jstate_change)
  stays valid. Drop Rongsong's duplicate unconditional re-PEND case.
- scheduler.c: port sched_requeue_pend_job (bug-1494 class) adapted to
  sched_get_job(&sched->core, ...) signature.
- types.h: re-add xce_sched_queue_t.dispatchwindow (our time-window PEND
  reason field) dropped when taking Rongsong's types.h.
- models/jobs.go: re-add JobSubmitResponse.JobIDHuman (human job id).
- worker controller_connector.c: our bug-1509 resend_job_status on rejoin
  success was already ported; add test stub.
- tests: adapt dispatch-fail test (code=0 → no re-PEND); fix sched_get_job
  signature in white-box stubs.

Build passes (nix build .#xce). Unit + live-cluster verification pending.

Co-Authored-By: Claude <noreply@anthropic.com>

src/frontend/apiserver/models/jobs.go

@@ -703,6 +703,7 @@ type ShellLimit struct {
 
 type JobSubmitResponse struct {
 	JobID     int64  `json:"jobid"`
+	JobIDHuman string `json:"job_id_human"` // cluster-apiserver-timestamp-seq, for xce_test/xcecli cross-reference
 	QueueName string `json:"queue_name"`
 	Message   string `json:"message"`
 }