Split _preflight_codex_input_items into per-item-type helpers over a
_PreflightCtx; streaming assembly moves into _CodexResponseAssembler with a
per-event dispatch table; run_codex_app_server_turn loses its session/usage/
interrupt regions to _ensure_codex_session/_finish_codex_turn/
_consume_user_interrupt/_queue_token_counts (counts built lazily so stub agents
without a session DB are never touched). Responses input/normalization and
stream callback order verified byte-identical against merge-base.
