The Kimi web-search adapter posts to the Kimi Code endpoint
(api.kimi.com/coding/v1/search) but resolved and advertised Moonshot
Open Platform credentials (moonshot provider, MOONSHOT_API_KEY,
api.moonshot.ai). Those are a different credential system, so a valid
Open Platform key was rejected with 401 and the preferred provider
silently fell back to another engine.
resolveKey() and isAvailable() now use kimi-code credentials only
(explicit MOONSHOT_SEARCH_API_KEY / KIMI_SEARCH_API_KEY overrides or a
stored kimi-code login), and the missing-credential error and provider
metadata name the Kimi Code requirement.
Fixes#5762