Add a device authorization grant flow (RFC 8628) for the kimi-coding
provider, mirroring the official Kimi Code CLI: device authorization and
token polling against https://auth.kimi.com, refresh-token rotation with
retry/backoff, and Bearer auth via toAuth headers. The provider now
offers 'Sign in with Kimi Code' alongside the existing KIMI_API_KEY
auth. Host is overridable via KIMI_CODE_OAUTH_HOST / KIMI_OAUTH_HOST.
Co-authored-by: Mario Zechner <badlogicgames@gmail.com>
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