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.