ML-21: Classify API errors as transient vs permanent

lib/brave_search/api/error_response.ex

@@ -0,0 +1,60 @@
+defmodule BraveSearch.API.ErrorResponse do
+  @moduledoc """
+  Structured error response for Brave Search API calls.
+
+  Brave Search returns a consistent JSON envelope on errors:
+
+      %{"type" => "ErrorResponse",
+        "error" => %{"status" => 422, "code" => "SUBSCRIPTION_TOKEN_INVALID",
+                     "detail" => "...", "meta" => %{...}}}
+
+  HTTP status codes are the primary classifier (429 for rate limit, 5xx for
+  server errors). Brave uses 422 for most validation failures including
+  authentication errors that other APIs would return as 401.
+  """
+
+  @behaviour MusicLibrary.ErrorResponse
+
+  alias MusicLibrary.HttpError
+
+  @type t :: %__MODULE__{
+          status: integer() | nil,
+          code: String.t() | nil,
+          message: String.t() | nil,
+          kind: HttpError.kind(),
+          body: term()
+        }
+
+  defstruct [:status, :code, :message, :kind, :body]
+
+  @spec from_response(Req.Response.t() | map()) :: t()
+  def from_response(%{status: status, body: body} = _response) do
+    %__MODULE__{
+      status: status,
+      code: extract_code(body),
+      message: extract_message(body),
+      kind: HttpError.default_kind(status),
+      body: body
+    }
+  end
+
+  @impl MusicLibrary.ErrorResponse
+  @spec retryable?(t()) :: boolean()
+  def retryable?(%__MODULE__{kind: kind}) when kind in [:rate_limit, :server_error, :timeout],
+    do: true
+
+  def retryable?(%__MODULE__{}), do: false
+
+  @impl MusicLibrary.ErrorResponse
+  @spec retry_delay_seconds(t()) :: pos_integer()
+  def retry_delay_seconds(%__MODULE__{kind: :rate_limit}), do: 60
+  def retry_delay_seconds(%__MODULE__{kind: :server_error}), do: 30
+  def retry_delay_seconds(%__MODULE__{kind: :timeout}), do: 10
+  def retry_delay_seconds(%__MODULE__{}), do: 30
+
+  defp extract_code(%{"error" => %{"code" => code}}) when is_binary(code), do: code
+  defp extract_code(_), do: nil
+
+  defp extract_message(%{"error" => %{"detail" => msg}}) when is_binary(msg), do: msg
+  defp extract_message(_), do: nil
+end