Move CreateModel, BuildReasoningOptions, ReasoningBudgetTokens and
related types from internal/agent to internal/models as NewSDKChatModel,
SDKModelConfig, etc. This eliminates duplicate ClientType constants and
centralises all Twilight AI SDK instance creation in a single package.
NewSDKEmbeddingModel now accepts a clientType parameter and dispatches
to the native Google embedding provider for google-generative-ai,
instead of always using the OpenAI-compatible endpoint.
Acbox