Document request validation workflow

examples/go-library/prepare/main.go

@@ -48,12 +48,16 @@ func main() {
 		exit(fmt.Errorf("required note input was not discovered before preparation"))
 	}
 
-	prepared, err := engine.Prepare(context.Background(), promptkit.RunRequest{
+	request := promptkit.RunRequest{
 		PromptID: "example.prepare",
 		Inputs: map[string]promptkit.ArtifactRef{
 			"note": promptkit.Inline("Ada finished the migration review."),
 		},
-	})
+	}
+	if err := engine.ValidateRequest(context.Background(), request); err != nil {
+		exit(err)
+	}
+	prepared, err := engine.Prepare(context.Background(), request)
 	if err != nil {
 		exit(err)
 	}