When /model is called with no arguments in the interactive CLI, open a
two-step prompt_toolkit modal instead of the previous text-only listing:
1. Provider selection — curses_single_select with all authenticated providers
2. Model selection — live API fetch with curated fallback
Also fixes:
- OpenAI Codex model normalization (openai/gpt-5.4 → gpt-5.4)
- Dedicated Codex validation path using provider_model_ids()
Preserves curses_radiolist (used by setup, tools, plugins) alongside the
new curses_single_select. Retains tool elapsed timer in spinner.
Cherry-picked from PR #7438 by MestreY0d4-Uninter.
