fix: recover codex stream when terminal output is null

[qiankemeng]

Summary

• Recover Codex streaming responses when the terminal output item has null content.
• Avoid a 'NoneType' object is not iterable failure during final response parsing.
• Add regression coverage for null terminal output handling.

Test Plan

• python -m py_compile agent/codex_runtime.py tests/run_agent/test_run_agent_codex_responses.py
• python -m pytest tests/run_agent/test_run_agent_codex_responses.py -q

[alt-glitch]

Duplicate of #32884 — same Codex null terminal output recovery. Root cause: #11179.

[teknium1]

Closing as duplicate — the Codex null-output fix has been merged via #32963 (cherry-picked from @carltonawong's PR #32890, the one Gille reviewed). Thanks for jumping on the outage so quickly; appreciate the help. Closes #11179.