End a v2 conversation through the close pipeline (admin)

Admin-triggered runtime end-session.

Distinct from POST /v1/sessions/{id}/close which only flips the DB status. This drives the full Runtime.end_session pipeline (AOP cleanup, memory flush, OTel session.end span, triggered_by=admin).

session_id is required: Runtime.from_authenticated falls through to get_or_create_active_session when it's missing, which would mint a fresh ACTIVE session purely to end it on the same request. Requiring the caller to name the target session prevents that ghost-session path and forces a clear 422 instead of a misleading 200.

farewell_text is intentionally omitted: the runtime ignores it under PersistStrategy.PER_TURN (used by every HTTP channel today), so accepting it would silently drop the value. Callers that want a user-visible farewell should send a final POST /v1/runtime/turn with the goodbye message before calling this endpoint.

Universal post-turn snapshot returned by Runtime.run_turn.

Every channel sees the same fields:

• session_id: the session this turn ran against. Always populated when a real Runtime produced the result (the caller may have passed session_id=None and asked the runtime to resolve / create the active session — this is how it learns which one was used). Optional in the type for the benefit of test fixtures that mint TurnResults by hand without a Runtime.
• messages: 1 message for API/voice; N (Plan 5) for SMS chunked.
• status: snapshot of Runtime._status at end-of-turn. The caller uses this to decide whether to call end_session.
• handoff: populated when AOP reached a HandoffNode. The dict shape mirrors the existing AOPExecutionResult.handoff_config for forward-compat — Plan 3 does not re-type it.
• error: when AOP execution fails, this is the AOPExecutionResult.execution_error string the pipeline stamped (e.g. "node_execution_failed: ValidationError: ...", "routing_error: <node_id>", "unknown_node_type"); falls back to the legacy "max_steps_exceeded" label only when the pipeline didn't attach a reason. repr(exc) for an uncaught exception in _execute. None on success / handoff / policy-violation.
• model_used / token_count / latency_ms: aggregate metrics surfaced for telemetry; None when the turn made no LLM call.