feat: add ClosedState enum ported from Go client.go

src/NATS.Server/ClosedState.cs

@@ -0,0 +1,52 @@
+// Ported from Go: server/client.go:188-228
+
+namespace NATS.Server;
+
+/// <summary>
+/// Reason a client connection was closed. Stored in connection info for monitoring
+/// and passed to close handlers during connection teardown.
+/// </summary>
+/// <remarks>
+/// Values start at 1 (matching Go's <c>iota + 1</c>) so that the default zero value
+/// is distinct from any valid close reason.
+/// </remarks>
+public enum ClosedState
+{
+    ClientClosed = 1,
+    AuthenticationTimeout,
+    AuthenticationViolation,
+    TLSHandshakeError,
+    SlowConsumerPendingBytes,
+    SlowConsumerWriteDeadline,
+    WriteError,
+    ReadError,
+    ParseError,
+    StaleConnection,
+    ProtocolViolation,
+    BadClientProtocolVersion,
+    WrongPort,
+    MaxAccountConnectionsExceeded,
+    MaxConnectionsExceeded,
+    MaxPayloadExceeded,
+    MaxControlLineExceeded,
+    MaxSubscriptionsExceeded,
+    DuplicateRoute,
+    RouteRemoved,
+    ServerShutdown,
+    AuthenticationExpired,
+    WrongGateway,
+    MissingAccount,
+    Revocation,
+    InternalClient,
+    MsgHeaderViolation,
+    NoRespondersRequiresHeaders,
+    ClusterNameConflict,
+    DuplicateRemoteLeafnodeConnection,
+    DuplicateClientID,
+    DuplicateServerName,
+    MinimumVersionRequired,
+    ClusterNamesIdentical,
+    Kicked,
+    ProxyNotTrusted,
+    ProxyRequired,
+}