Delete the stale literal `_PROVIDER_MODELS["ai-gateway"]` (gpt-5,
gemini-2.5-pro, claude-4.5 — outdated the moment PR #13223 landed with
its curated `AI_GATEWAY_MODELS` snapshot) and derive it from
`AI_GATEWAY_MODELS` instead, so the picker tuples and the bare-id
fallback catalog stay in sync automatically. Also fixes
`get_default_model_for_provider('ai-gateway')` to return kimi-k2.6
(the curated recommendation) instead of claude-opus-4.6.