Add the legacy `models.providers.*.api: "openai"` → `"openai-completions"`
migration to doctor's Current migrations list, and note the gateway startup
behavior that skips providers with future or unknown api enum values instead
of failing closed.
Traces to:
- 6a7980e984 fix(doctor): migrate legacy OpenAI provider api
- 147f4f50f5 fix(gateway): skip stale model provider api entries