feat(codex): add GPT-5.6 support

[sjungwon03]

Summary

• add the GPT-5.6 family to the ChatGPT/Codex model selector and FastAPI allowlist: gpt-5.6, gpt-5.6-sol, gpt-5.6-terra, and gpt-5.6-luna
• keep gpt-5.6 as the default Codex model and preserve fallback behavior for unsupported IDs
• cover all GPT-5.6 model IDs in frontend and backend regression tests

Verification

• uv run pytest tests/unit -q — 452 passed
• npm test — 47 passed
• npm run build — passed
• npm run lint — passed with 101 pre-existing warnings and no errors
• git diff --check — passed

AI disclosure

Implementation was AI-assisted, manually reviewed, and independently diff-reviewed before submission.

Closes #764

[CLAassistant]

All committers have signed the CLA.

[sudipnext]

Hey, thanks for putting this together! Just one quick catch: we actually can't use the bare gpt-5.6 model ID with this integration. Let's stick to just the specific family models: gpt-5.6-sol, gpt-5.6-terra, and gpt-5.6-luna.