Behaviour for WebSocket subscription pattern implementations.

Each pattern module converts a list of pre-formatted channel strings (or per-pattern objects) into the exchange-native subscribe frame. Frame encoding (Jason) happens at the WS boundary in Bourse.WS.subscribe/3, not inside pattern modules — modules return plain maps.

Return shape

subscribe/2 and unsubscribe/2 return map() | [map()] | {:error, term()}.

• Most exchanges accept an array of channels in a single frame → return a map().
• HTX (:sub_subscribe) requires one frame per channel → return a [map()] with one frame per channel.
• Upbit-style custom (config[:custom_type] == "array_format") also returns a [map()].
• Implementations that enforce an input-shape contract may return {:error, term()} (e.g. :multiple_maps_not_supported or :mixed_channel_types from EventSubscribe / MethodParams). Bourse.WS.Subscription.build_subscribe/3 passes these through to the caller verbatim rather than wrapping them in {:ok, _}.

Bourse.WS.subscribe/3 handles both frame shapes by iterating the list when present and sending one ZenWebsocket.Client.send_message/2 per frame.

Channel formatting is the caller's responsibility

Pattern modules do not format channel strings from unified symbols. Callers pass pre-formatted channels like "tickers.BTCUSDT" or "market.btcusdt.ticker". A future task may introduce spec-driven channel templates; this behaviour is deliberately narrow.

Implementing a Pattern

defmodule Bourse.WS.Subscription.OpSubscribe do
  @behaviour Bourse.WS.Subscription.Behaviour

  @impl true
  def subscribe(channels, config) do
    %{
      (config[:op_field] || "op") => "subscribe",
      (config[:args_field] || "args") => channels
    }
  end

  @impl true
  def unsubscribe(channels, config) do
    %{
      (config[:op_field] || "op") => "unsubscribe",
      (config[:args_field] || "args") => channels
    }
  end
end