A modern MCP subscription registration.
subscription_id is the client-chosen JSON-RPC request ID used on the wire. token is a distinct unguessable internal identifier and is never sent to the peer.
subscription_id
token
@type t() :: %ExMCP.Server.Subscriptions.Entry{ expires_at: integer(), filter: map(), listener_pid: pid(), principal_id: String.t() | nil, subscription_id: String.t() | integer(), tenant_id: String.t() | nil, token: String.t(), transport_ref: pid() }