@spec format_error(term()) :: String.t()

Formats error reasons from request preparation into human-readable messages.

@spec prepare_body(term(), [{String.t(), String.t()}], term(), keyword()) ::
  {:ok, [{String.t(), String.t()}], iodata()} | {:error, term()}

Prepares the request body and updates headers as needed.

For multipart requests (with files), encodes as multipart/form-data. For regular requests, encodes body as JSON.

Returns {:ok, headers, body} or {:error, reason}.