Runtime Provider transport policy.
:auto prefers a per-key WebSocket connection and falls back to HTTP/SSE for
replay-safe transport failures. Ambiguous connection call loss is not proof of a
clean stream and therefore vetoes fallback. The fallback reuses the same HTTP
request body, so safe
prompt_cache_key metadata survives across transports. previous_response_id lives
only inside the WebSocket connection process and is discarded on fallback.