feat(component-failover-adoption): unify scheduler's dual connect paths into single pipeline

Replace scheduler's inline per-candidate loop and separate single-host
branch with one unified pipeline (mirrors worker's task 2.1 fix): build
exactly one validated candidate list, whether from --peers or synthesized
as a single candidate from the legacy default, then connect through the
shared xce_cluster_config_to_client_config() helper exactly once.

Adds sched_on_reconnect(), a rejoin callback that only sets a flag and
returns immediately (no I/O, no xcecli_* calls) to avoid the self-deadlock
worker hit in task 2.3 — registered via xcecli_set_reconnect_handler()
after xcecli_start(), since xcecli_start() resets any handler registered
earlier back to NULL.

Adds test_scheduler_connect.c: behavioral tests confirm no-peers and
single-candidate configs produce identical connect log sequences and that
unreachable candidates fail fast without retry loops; structural tests
source-scan scheduler.c to confirm a single xcecli_ctx_init() call site, a
single xcecli_join() call site, and a rejoin callback body free of I/O and
xcecli_* calls.

Live-validated against a 3-node HA controller cluster: a scheduler
configured with the standby listed first in --peers fails over via
xcecli's built-in retry-next mechanism to the primary and joins
successfully.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FYSEeLmzjtD4RLDuUDhLoK

src/scheduler/testsuite/meson.build

@@ -761,6 +761,36 @@ sched_repl_equivalence_test = executable(
     dependencies: [scheduler_test_deps, libedit_dep],
     install: false)
 
+# Tests for the scheduler's unified candidate-list connect/rejoin pipeline
+# (component-failover-adoption, task 3.1, boundary: SchedulerControllerClient).
+# Unlike the white-box #include-the-.c-file tests above, this target links
+# the REAL scheduler_srcs (everything sched_init() needs -- core.c,
+# schedobj.c, resource.c, plugin.c, the picker/allocator plugins, etc.) and
+# calls the real, public sched_init()/sched_fini(): scheduler.c's own
+# dependency surface is too large for whole-file inclusion (same "too many
+# externals to stub" reasoning sched_picker_core_loop_boundary_test's own
+# comment already gives for not #include-ing scheduler.c), so this avoids
+# that by linking the real objects instead of textually including one.
+# scheduler_srcs (defined in the parent meson.build) minus src/main.c (this
+# test file supplies its own main()) -- built via a loop rather than a
+# hand-duplicated list so it can never drift from the production source list.
+scheduler_connect_test_srcs = ['test_scheduler_connect.c']
+foreach f : scheduler_srcs
+  if f != 'src/main.c'
+    scheduler_connect_test_srcs += ['../' + f]
+  endif
+endforeach
+
+sched_scheduler_connect_test = executable(
+    'sched_scheduler_connect_test',
+    scheduler_connect_test_srcs,
+    include_directories:[incdir],
+    dependencies: [scheduler_deps, unity_dep],
+    c_args: ['-DXCE_SCHEDULER_C_PATH="'
+             + (meson.current_source_dir() / '..' / 'src' / 'scheduler.c')
+             + '"'],
+    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',
@@ -863,3 +893,6 @@ test('scheduler repl snapshot params test',
 test('scheduler repl equivalence test',
      sched_repl_equivalence_test, suite: 'scheduler',
      env: ['ASAN_OPTIONS=detect_leaks=0'])
+test('scheduler connect pipeline test',
+     sched_scheduler_connect_test, suite: 'scheduler',
+     env: ['ASAN_OPTIONS=detect_leaks=0'])