List state codes

Start an Esign session using a previously saved template. The template contributes `file_id`, signer elements, and signer defaults; the request contributes top-level Esign options plus runtime signer data in the **same order** as the template's `signers` array. - Runtime signer count must match the template signer count. - `element_values` overrides template element `value`s at runtime by template `field_id`; position and type still come from the template. - Template signer defaults are merged with runtime signer input before the normal `initialize-esign` validation flow runs. - `full_name` should be supplied, `reason` must be supplied unless saved in the template, and at least one of `email` or `mobile` must be present. - This endpoint creates a real Esign flow, just like `initialize-esign`.