Sends periodic HeartbeatRequests for a joined consumer-group member.

The Manager starts this process linked and trapping exits. Every failure this process can catch is turned into a structured {:shutdown, reason} exit that the Manager maps to a rejoin or a terminal stop. The one failure it cannot catch — :kill (untrappable; no callback runs) — reaches the Manager as a plain :killed EXIT.

Lifecycle

Manager ──start_link (linked)──▶ Heartbeat
                                    │  loop: every heartbeat_interval
                                    ▼
                      send HeartbeatRequest to the broker
                                    │
                 :no_error ◀────────┴────────▶ error / crash
                    │                              │
              keep ticking                exit {:shutdown, reason}

exit reason                                  ─▶ Manager action
────────────────────────────────────────────────────────────────
:rebalance                                   ─▶ rejoin
{:rejoin, :illegal_generation}               ─▶ rejoin, keep member_id
{:rejoin, :unknown_member_id}                ─▶ rejoin, reset member_id
{:error, recoverable}  (e.g. :timeout)       ─▶ rejoin
{:error, non-recoverable} | {:terminal, _}   ─▶ terminal stop
:killed  (uncatchable)                       ─▶ rejoin

See KafkaEx.Consumer.ConsumerGroup.Manager for the EXIT-clause handling.