refactor(core): move heartbeat/liveness logic into core behind remote…

[LiamCarPer]

refactor(core): move heartbeat/liveness logic into core behind remote-adapter feature

Motivation

The postgres and mongodb adapters independently implement the same
heartbeat/liveness logic: the same nodes_liveness: Mutex<Vec<(Uid, Instant)>>
field, the same interval-based heartbeat job, the same
emit_heartbeat/emit_init_heartbeat helpers, the same recv_heartbeat
handler (~130 LOC duplicated verbatim, only the error type differs) and the
same server_count dead-node pruning. This duplication makes maintenance
harder and is contrary to the goal of #727 (share more code between adapters).

Solution

Move the heartbeat/liveness logic into socketioxide-core behind the existing
remote-adapter feature flag:

• HeartbeatTracker: wraps nodes_liveness + hb_timeout; on_heartbeat
  (update-or-push, returns whether an InitHeartbeat reply is due),
  server_count (prune dead nodes + self), is_alive.
• HeartbeatSender trait: uid, send_req, with default
  emit_heartbeat, emit_init_heartbeat and recv_heartbeat implementations.
• heartbeat_loop helper.

Postgres and mongodb now keep only a thin HeartbeatSender impl (their
adapter-specific send_req + uid) plus a HeartbeatTracker field.
No behavior or wire-format change. Redis is unaffected: it has no heartbeat
(liveness is pub/sub subscriber counting via num_serv).

Closes #767

[codspeed-hq]

Merging this PR will not alter performance

✅ 87 untouched benchmarks

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_{Comparing LiamCarPer:refactor/move-heartbeat-to-core (c0b003a) with main (144497d)}

[LiamCarPer]

Totodore I wanted to say Thanks for the patience, really appreciate you letting me take on this refactor. Happy to iterate on any feedback.