Fix Codex Responses streaming crash on null-output snapshots

[Wondermonger-daydreaming]

Problem

Every gauss chat turn using the openai-codex provider (i.e. a ChatGPT subscription, chatgpt.com/backend-api/codex) aborts immediately with:

TypeError: 'NoneType' object is not iterable

Root cause

The ChatGPT Codex Responses backend deviates from the public api.openai.com Responses API:

• It emits streaming SSE snapshot events with output=None (the public API uses output=[]).
• The OpenAI SDK's responses.stream() accumulator calls parse_response() on every event, which does for output in response.output: — this raises TypeError: 'NoneType' object is not iterable on the first event (response.created) of every Codex turn.
• _run_codex_stream only falls back on a RuntimeError (missing response.completed), so this TypeError propagates and the request is treated as a non-retryable client error.
• The backend also leaves .output None on the terminal response.completed event — the completed items arrive only via response.output_item.done, and text only via response.output_text.delta. So even the manual fallback returned a response whose .output / .output_text access crashed downstream.

Note: this is a backend deviation, not documented OpenAI behavior — bumping the openai SDK does not fix it, and guarding the SDK's parse_response yields empty text (the accumulator has no populated output to merge deltas onto).

Fix

Two surgical changes in run_agent.py:

1. _run_codex_stream — catch the accumulator TypeError and route to the existing manual create(stream=True) fallback.
2. _run_codex_create_stream_fallback — collect response.output_item.done items during iteration and reconstruct the terminal response's .output via a new _reconstruct_codex_output() helper (no-op when .output is already populated, so non-Codex Responses backends are unaffected).

Verification

Tested against a live ChatGPT (Codex) subscription:

• ✅ Default model
• ✅ Pinned gpt-5.3-codex
• ✅ Tool-using turns (read_file → correct result)

Each path previously failed with the TypeError; all now return correctly.