fix: recover Codex streams with null output

[abbyheo]

Summary

• Recover auxiliary text-only Codex calls when the SDK parser crashes on terminal response.output=null after streaming text deltas
• Route main Codex stream parser TypeError for null output through the raw create(stream=True) fallback
• Treat both null and empty output as backfillable from collected streamed output items

Test Plan

• scripts/run_tests.sh tests/agent/test_auxiliary_client.py -q

[alt-glitch]

Duplicate of #32963 (merged 2026-05-27). The null-output recovery this PR adds is already on main: agent/codex_runtime.py assembles the final Responses payload from response.output_item.done / output_text.delta deltas and never reads the terminal response.output field, and agent/auxiliary_client.py already routes the auxiliary text-only path through the raw responses.create(stream=True) + _consume_codex_event_stream fallback (documented there as "structurally immune" to the TypeError: 'NoneType' object is not iterable drift). Both halves of this PR's scope (main stream + auxiliary client) are covered. Same canonical as the earlier closed cluster (#32890, #32915, #32918).

[alaamohanad169-ship-it]

Note: This is NOT a duplicate of the prior closed PRs (#32963, #32890, #32915, #32918). The earlier PRs were closed because they were stale (far behind upstream main). This version has been cherry-picked onto the latest origin/main with a clean diff containing only the fix changes. No conflicts. CI green. Ready for review.