feat(scheduler-controller-msg-gap): route notify job-update to in-place sched_update_existing_job (task 5.6, bug-765 G-8+G-4)

The notify schedUpdateJob path shared XCE_SCHED_OP_UPDATE_JOB with the
dataset bulk-load, so the dequeue routed both to sched_update_jobs()
(create/replace) — a notify update to an existing job corrupted/duplicated
live job state and never returned -ENOENT (Req 3 UB-3.1/UB-3.2 unmet;
sched_update_existing_job was dead code copying only state).

- Add distinct op XCE_SCHED_OP_UPDATE_JOB_FIELDS (+ jobfieldsupdate union),
  enqueued only by the notify handler, dequeued to sched_update_existing_job.
- Move sched_update_existing_job to schedobj.c; update all mutable fields
  (state, priority, queue, resreq) in place; -ENOENT unknown, -EINVAL NULL.
- Dataset bulk-create path unchanged. Killer frees the new payload.
- New white-box test sched_job_update_test (3/3); regression sched_test 15/15.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/scheduler/testsuite/meson.build

@@ -11,3 +11,12 @@ scheduler_test = executable('sched_test',
                             include_directories:[incdir],
                             dependencies: [scheduler_test_deps],
                             install: false)
+
+# White-box test for the notify single-job in-place update path
+# (sched_update_existing_job). It #includes ../src/schedobj.c directly, so it
+# is a standalone TU with its own main() — built as a separate executable.
+sched_job_update_test = executable('sched_job_update_test',
+                                   ['test_job_update.c'],
+                                   include_directories:[incdir],
+                                   dependencies: [scheduler_test_deps],
+                                   install: false)