Start or resume a Sunburst chat session scoped to one Sunburst-guided team onboarding
step. Admin access only. Starts a session that can see only that step's onboarding
tool(s), and records the session on the team's onboarding progress (sunburst_session_id,
status in_progress, started_ts set the first time this step is started).

If the step already has a sunburst_session_id and start_fresh is not true, this resumes
that session (loads the prior response chain and continues it) instead of restarting from
the canned prompt. Otherwise (first time starting the step, or start_fresh=true) it loads
the step's canned prompt and starts a brand-new session, overwriting any prior
sunburst_session_id. Returns a session_id and the initial thread, identical to
POST /api/v3/sunburst/chat.