Native Codex app-server turns with caller-implemented dynamic tools.
[Codex::start_turn] preserves the standard text-and-dynamic-tool flow.
[Codex::start_image_turn] is a separate fresh, ephemeral, tool-free flow
that sends caller-owned image bytes as inline data URLs.
[AgentTurn] yields exact client-to-Codex JSONL records, dynamic tool
calls, and the completed turn. For an image turn, the turn/start
[AgentEvent::ProviderInput] contains the Base64-encoded image bytes and
must be handled as media-bearing data.