feat(registry): add client model support check for executor filtering

- Introduced `ClientSupportsModel` function to `ModelRegistry` for verifying client support for specific models.
- Integrated model support validation into executor candidate filtering logic.
- Updated CLIProxy registry interface to include the new support check method.

sdk/cliproxy/model_registry.go

@@ -11,6 +11,7 @@ type ModelRegistry interface {
 	UnregisterClient(clientID string)
 	SetModelQuotaExceeded(clientID, modelID string)
 	ClearModelQuotaExceeded(clientID, modelID string)
+	ClientSupportsModel(clientID, modelID string) bool
 	GetAvailableModels(handlerType string) []map[string]any
 }