Get or create a v2 conversation

Get-or-create a v2 conversation (Phase F2) — CHAT ONLY (D4/ADR-007).

The generic create route creates chat conversations only: sms / voice / email creation stays adapter-owned (/v1/sms/send, voice dispatch, the email pipeline). channel is therefore not a request field (forced server-side), and origin / channel_subtype are gone — both are server-assigned (ADR-005 / ADR-004).

Bind to exactly one of a single Assistant (assistant_id) or a Team (team_id + team_revision_id) — the single_xor_team CHECK on conversations. assistant_revision_id is optional; when omitted it is resolved from the agent's active revision and pinned at bind time.