Tighten prepared execution credential handling

engine.go

@@ -149,12 +149,14 @@ type engineOptions struct {
 	artifactSource  bool
 }
 
-// WithLLMClient replaces the built-in model client used by [Engine.Run].
+// WithLLMClient replaces the built-in model client used by [Engine.Run] and
+// [Engine.RunPrepared].
 //
 // A nil client makes NewEngine fail with ErrInvalidConfig. The Engine schedules
 // Generate calls according to the selected backend's capacity policy, but the
 // client may still be called concurrently across different backend pools or for
-// unlimited backends. The client is not used by [Engine.Prepare].
+// unlimited backends. The client is not used by [Engine.Prepare] or
+// [Engine.PrepareExecution].
 func WithLLMClient(client LLMClient) Option {
 	return optionFunc(func(options *engineOptions) error {
 		if client == nil {