chore(models): update GPT-5.6 context limits in registry metadata

- Raise GPT 5.6 Sol/Terra/Luna `context_length` values to `921000` in `internal/registry/models/models.json`.
- Update matching Codex client model settings to `context_window: 272000` and `max_context_window: 921000` in `internal/registry/models/codex_client_models.json`.