feat(codex): support `max-context-length` overrides for configured models

- Add optional `max-context-length` model configuration across supported provider model types and expose it via `GetMaxContextLength`.
- Propagate the override into model metadata so Codex/client model catalog responses honor the configured value (`context_window`, `max_context_window`, and `max_context_length`).
- Update example configuration with documented usage of the new option.

Closes: #4728