feat(ha): reference-counted ha_elect_ctx_t lifecycle (ha-elect-ctx-lifecycle)

Replace the deliberate election-carrier leak with an atomic refcount:
a bias reference held by ha->elect_ctx plus one reference per captured
vote endpoint, released via a new endpoint_t.on_fini teardown-completion
hook (fired once, before ep->data is freed). Both real-election and
PreVote capture sites (raft_cb_send_vote_request,
raft_cb_send_pre_vote_request) increment on capture; both publish sites
(ha_start_election_async, ha_start_prevote_async) now atomic_exchange
the outgoing carrier and release it instead of leaking or freeing
outright, so a still-outstanding vote endpoint keeps the superseded
carrier alive until its own teardown completes.

Verified: 758+ controller tests green, isolated LeakSanitizer run shows
zero ha_elect_ctx_t leaks across many election/PreVote rounds, no
premature-free/UAF under supersession, and a live 10-round HA soak
(36 jobs, zero loss, all 3 nodes served as primary).

src/lib/meson.build

@@ -173,6 +173,7 @@ libxce_test_src = ['testsuite/test.c',
                    'testsuite/test_cluster_parse.c',
                    'testsuite/test_libxce_retry.c',
                    'testsuite/test_netio_on_connected.c',
+                   'testsuite/test_netio_on_fini.c',
                    'testsuite/test_netio_arm_failure.c',
                    'testsuite/test_netio_close_propagation.c',
                    'testsuite/test_netio_bounded_connect.c',