API token create/edit ignores the chosen expiry

## What

creating/editing an API token ignores the chosen expiry and always applies the system default token duration.

## Want

- Creating/editing an API token ignores the chosen expiry
- Always applies the system default token duration

## Ideas this came from

- [#14093 Expiry date for API tokens not honored](https://github.com/goauthentik/authentik/issues/14093) — The expiry date input is ignored when creating or modifying an API token. To Reproduce Steps to reproduce the behavior: 1. Create a token, (I chose API Token) 2
- [#19045 Bug when creating an API token](https://github.com/goauthentik/authentik/issues/19045) — When creating an API token, it is not possible to choose a custom expiry date. The expiry date taken into account is always the one in System Settings Default t

Start by reproducing API token creation and editing with a custom expiry, then trace where the chosen expiry is received and where the system default duration is applied. Done means both create and edit operations honor the selected expiry instead of replacing it with the system default; the issue names no files or tests.

Written by the indexing model from the issue text.