A refused initial connect (or a failed Sparkplug establish) now disposes the
MqttConnection it built. Every lifecycle operation takes a generation (initialize
inside the gate; shutdown/dispose before waiting for it): a wedged initialize that
completes after a timed-out ShutdownAsync's ungated teardown releases its session
instead of publishing it, and a superseded operation's health/host-state writes are
refused. TeardownAsync captures every field before disposing so an ungated teardown
never reaches into a later initialize's state.
Refs #744
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
43f45bb978