Add EventBuilder static class to EventTypes.cs with helpers for constructing
fully-populated ConnectEventMsg, DisconnectEventMsg, AccountNumConns, and
ServerStatsMsg. Also add RemoteServerShutdownEvent, RemoteServerUpdateEvent,
LeafNodeConnectEvent, and LeafNodeDisconnectEvent advisory types.
Add FullEventPayloadTests.cs (10 tests) covering all builders, GenerateEventId
uniqueness, GetTimestamp ISO 8601 format, DataStats zero defaults, and
ConnectEventMsg JSON roundtrip.
Add JetStream advisory subject constants to EventSubjects and a lightweight
AdvisoryPublisher that publishes stream/consumer lifecycle events to
$JS.EVENT.ADVISORY.* subjects without depending on InternalEventSystem
directly (testable via Action delegate injection).
Register VARZ, HEALTHZ, SUBSZ, STATSZ, and IDZ request-reply handlers
on $SYS.REQ.SERVER.{id}.* subjects and $SYS.REQ.SERVER.PING.* wildcard
subjects via InitEventTracking. Also excludes the $SYS system account
from the /subz monitoring endpoint by default since its subscriptions
are internal infrastructure.
