## Assistant Chat: HTTP 417 from `check_generation` treated as error instead of pending state

**Relates to:** #3978

### Description

The Nextcloud server intentionally returns HTTP `417 Expectation Failed` from the `check_generation` endpoint as a signal that the AI task is still processing (pending). However, the new iOS Assistant Chat client (`NCAssistantChatModel.swift`, introduced in #3978) treats any `4xx` response as a fatal error, 