getCallbackUrl
Returns a per-operation callback URL by appending operation to SampleConfig.callbackBaseUrl, giving each initiation endpoint its own callback path (e.g. <base>/requesttopay, <base>/deposit). Returns an empty string when no base URL is configured, so the X-Callback-Url header is omitted entirely until a callback base URL is set.
Parameters
operation
The initiation endpoint's path segment (e.g. "requesttopay", "deposit").