Cap on body length when surfacing a provider’s non-2xx response into
CloudAiError::Status. The whole point is that this string flows back
through tauri::command -> Result<_, String> to the renderer (and into
logs along the way) - providers occasionally return multi-KB debug
bodies, and an unbounded body inflates log lines and the renderer
surface for no extra signal. The first ~1KB is enough to identify the
failure class; anything longer is truncated with an explicit marker so
the operator knows there was more.
Trim and length-cap a raw provider error body before stuffing it into
CloudAiError::Status. Idempotent and safe on UTF-8: truncates at the
last char boundary <= MAX_ERROR_BODY_LEN.