feat: fill scheduler controller message gaps

Wire up 6 previously-stubbed message handlers in the scheduler:
resource dataset, resval dataset, job update notify, queue update
notify, resval update notify, and job rusage notify. Remove the
unreachable pushmsg branch (vtask_entry rewrites all pushmsg to
datamsg before the handler fires).

Changes by component:
- helper.h/helper.c: add xce_notify_event_name() for symbolic log names
- scheduler.h: add 5 new xce_sched_op_t values and matching union fields
- plugin.h/plugin.c: add plugin_job_rusage_hook_t and plugin_job_rusage_update()
- schedobj.c: add sched_update_existing_queue() with NULL/-ENOENT guards
- scheduler.c: wire 5 new dequeue cases; add sched_update_existing_job()
  and static sched_op_name(); emit WARN log on handler failure
- handler.c: implement sched_handle_resources/resvals (dataset) and
  sched_handle_sched_update_{job,queue,resvals}/job_rusage (notify);
  remove dead sched_handle_jobpush/workerpush and pushmsg branch;
  update TRACE logs to use symbolic name helpers
- test.c: 5 new unit tests covering notify event names, enum values,
  and queue update NULL guard (15/15 pass)

src/scheduler/include/plugin.h

@@ -21,6 +21,8 @@ typedef int (*plugin_job_state_change_hook_t)(void*, xce_sched_job_t*,
 typedef int (*plugin_commit_decision_hook_t)(void*, xce_decision_t*);
 typedef int (*plugin_job_election_hook_t)(void*, xce_sched_job_t*);
 typedef int (*plugin_prepare_allocation_hook_t)(void*, xce_sched_ctx_t*);
+typedef int (*plugin_job_rusage_hook_t)(void*, xce_sched_job_t*,
+                                        xce_job_rusages_t*);
 
 /* initialize function of plugin */
 typedef int (*plugin_init_fn_t)(void*);
@@ -48,6 +50,7 @@ typedef struct {
     plugin_commit_decision_hook_t    commit_hook;
     plugin_job_election_hook_t       job_election_hook;
     plugin_prepare_allocation_hook_t prepare_allocation_hook;
+    plugin_job_rusage_hook_t         rusage_hook;
 } plugin_hooks_t;
 
 typedef struct {
@@ -76,6 +79,7 @@ extern int  plugin_end_session(void*);
 extern int  plugin_commit_decision(void*, xce_decision_t*);
 extern int  plugin_prepare_allocation(void*, xce_sched_ctx_t*);
 extern int  plugin_job_election(void*, xce_sched_job_t*);
+extern int  plugin_job_rusage_update(void*, xce_sched_job_t*, xce_job_rusages_t*);
 extern int  register_fcfs(void*);
 extern int  register_limit(void*);
 extern int  register_seqjob_allocator(void*);