Base for a per-session handshake subscriber.

use FixAlchemy.Session gives a GenServer that subscribes to the session's client for the message types it cares about and dispatches each to handle_fix/4. The defaults implement a plain FIX session: it subscribes to logon (A) and marks the session ready the moment logon is accepted.

A broker adapter overrides the callbacks with @impl FixAlchemy.Session to add its own login flow — credentials, account discovery, entitlements — while reusing everything else:

defmodule MyBroker.Session do
  use FixAlchemy.Session

  @impl FixAlchemy.Session
  def message_types, do: ["A", "BF", "BA"]

  @impl FixAlchemy.Session
  def handle_fix("A", _raw, meta, state) do
    FixAlchemy.Client.send_custom_message(meta.client, {"BE", credentials(state)})
    state
  end

  def handle_fix(type, raw, meta, state), do: super(type, raw, meta, state)
end