Set generated step types

<Warning> **Coming soon.** This endpoint will be available by early July 2026. </Warning>

<small>Requires the ai-sdr:write scope (or a broader one that includes it).</small>

Replaces the step channels the AI SDR is allowed to generate, and triggers regeneration of the sequence's steps using the new mix.

Rejected with 409 once the sequence has already acted on contacts (sent emails or LinkedIn messages, generated tasks, etc.).

Requires the AI SDR feature on the caller's team.