fix(scheduler): mid-round rejoin config push truncated live rounds (bug-1846 part 2)

sched_publish_retry_loop() calls sched_check_pending_rejoin() after every
successful publish -- inside the driver's decision-drain loop -- so after a
client-library reconnect the scheduler re-joins mid-round. The controller
answers every join (on_scheduler_join(), controller/sched.c) by re-pushing
config, workers and jobs. handler.c applied the config push synchronously
on the handler thread: sched_init_config() -> sched_init_queues() ->
builtin_fcfs on_queue_update(), which nulls the picker cursor when it
reinserts the queue the cursor rests on. next_job() then returned NULL and
the round ended with the rest of the queue never considered.

Evidence (scheduler-replay-20260923-085356.replay, temporary event-stream
diagnostics, reverted): 9 recorded rounds end after 11-12 decisions; every
one is followed by a workersPush + full 50-job jobsPush (the join's resync,
reporting just-dispatched jobs back as PEND), every full-length dispatching
round has none, and the config push never appears because it bypassed the
pending-update queue the recorder writes from -- which is also why replay
could not reproduce the truncation.

Fix (defer to the round edge, same direction as bug-1813): handler.c now
calls new sched_stash_config(), parking the Conf4Sched in new
xce_scheduler_t.pending_config under pending_config_lock (an older
unapplied push is freed). main.c's loop calls new
sched_apply_pending_config() before each round, running the unchanged
sched_init_config() strictly between sessions. sched_fini() frees a
never-applied push.

Tests:
- test_handlers.c: test_config_push_is_stashed_not_applied_on_handler_thread
  -- RED (sched_init_config called on the handler thread), GREEN after.
- new test_config_push_deferred.c (sched_config_push_deferred_test): real
  scheduler_req_handler() + real builtin_fcfs mid-session -- RED (next_job()
  returned NULL right after the push), GREEN (session walks the remaining
  jobs; configver unchanged until applied once between rounds).
- test_admin_request.c / test_job_array_expand.c / test_job_array_wire_e2e.c
  #include handler.c directly and get a link stub for sched_stash_config().
Full scheduler suite 87/87 (asan).

Not addressed: config pushes are still not written to a recording (a
replay spanning a config change won't see it); the ~100s reconnect churn
itself (see bug-1812/1813).

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

src/scheduler/testsuite/meson.build

@@ -2231,6 +2231,30 @@ test('scheduler limit host dedup test',
      sched_limit_host_dedup_test, suite: 'scheduler',
      env: ['ASAN_OPTIONS=detect_leaks=0'])
 
+# bug-1846 part 2: a config push (sent by the controller on every scheduler
+# join, including a rejoin triggered mid-round from the decision publish
+# path) must be deferred to the round boundary -- applying it on the handler
+# thread reset the fcfs cursor and ended the live round early. Real
+# scheduler_req_handler() + real builtin_fcfs; every scheduler source except
+# main.c.
+sched_config_push_deferred_test_srcs = ['test_config_push_deferred.c']
+foreach f : scheduler_srcs
+  if f != 'src/main.c'
+    sched_config_push_deferred_test_srcs += ['../' + f]
+  endif
+endforeach
+
+sched_config_push_deferred_test = executable(
+    'sched_config_push_deferred_test',
+    sched_config_push_deferred_test_srcs,
+    include_directories:[incdir],
+    dependencies: [scheduler_test_deps],
+    install: false)
+
+test('scheduler config push deferred test',
+     sched_config_push_deferred_test, suite: 'scheduler',
+     env: ['ASAN_OPTIONS=detect_leaks=0'])
+
 # scheduler-sim-live-allocation spec, task 5.2 (Req 2.1, 2.2, 2.3, 2.4): the
 # feature's concrete, original acceptance test (bug-1826) -- a real global
 # AND host-scoped limit configured in a real SnapshotEntry's Conf4Sched,