Add "ultra" reasoning level for Codex GPT-5.6 Sol and Terra, and expose
Max for Luna (Luna falls back Ultra to Max since it is not supported
upstream). Scoped to cx/ routes only; Kiro and generic OpenAI routing
unchanged.
Expose max in the Codex thinking dropdown for gpt-5.6-sol only (maps to
xhigh on wire; live probe rejected ultra). Include custom/kilo models
when computing provider thinking options so manually added gpt-5.6-sol
contributes its max level.
Co-authored-by: Cursor <cursoragent@cursor.com>