test(scheduler-controller-msg-gap): real handler/applier unit tests (bug-765 G-6)

The Task 9 tests were marked [x] but were stub/absent. Convert the
correctness-critical ones to real white-box tests, registered as meson tests
under --suite scheduler (leaks-off per convention):

- test_handlers.c (new) — #include ../src/handler.c; covers 9.2/9.3: each
  notify/dataset handler enqueues the correct op / skips NULL-empty / -EINVAL.
  Asserts the 5.6 fix (update_job -> UPDATE_JOB_FIELDS, not bulk UPDATE_JOB).
- test_job_update.c — add 9.5 (real sched_update_existing_queue, replacing the
  former local stub); already covers 9.4 (job) and 9.7 (rusage).
- meson.build — build + register all three scheduler test executables.

Suite GREEN: sched_test 15/15, sched_job_update_test 10/10,
sched_handlers_test 6/6; ASAN-clean.

Honest remaining: 9.6 dequeue worker-resvals APPLY is PARTIAL (enqueue tested;
apply is a dormant path, R-3); 9.8 is a documented compile-time absence check.

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

src/scheduler/testsuite/meson.build

@@ -20,3 +20,21 @@ sched_job_update_test = executable('sched_job_update_test',
                                    include_directories:[incdir],
                                    dependencies: [scheduler_test_deps],
                                    install: false)
+
+# White-box test for the notify/dataset enqueue handlers. It #includes
+# ../src/handler.c directly (enq_pend_update + the static handlers), so it is a
+# standalone TU with its own main() — built as a separate executable.
+sched_handlers_test = executable('sched_handlers_test',
+                                 ['test_handlers.c'],
+                                 include_directories:[incdir],
+                                 dependencies: [scheduler_test_deps],
+                                 install: false)
+
+# Register as meson tests so `meson test --suite scheduler` runs them all.
+# detect_leaks=0 matches the suite convention (fixtures are not torn down).
+test('scheduler test', scheduler_test, suite: 'scheduler',
+     env: ['ASAN_OPTIONS=detect_leaks=0'])
+test('scheduler job-update test', sched_job_update_test, suite: 'scheduler',
+     env: ['ASAN_OPTIONS=detect_leaks=0'])
+test('scheduler handlers test', sched_handlers_test, suite: 'scheduler',
+     env: ['ASAN_OPTIONS=detect_leaks=0'])