Add prepared execution lifecycle to the runner

internal/jsonvalue/jsonvalue.go

@@ -18,13 +18,19 @@ type visit struct {
 	ptr uintptr
 }
 
+// Copy validates and deeply copies a JSON-compatible value while preserving
+// compatible concrete map, slice, array, scalar, and number types.
+func Copy(src any) (any, error) {
+	return copyValue(reflect.ValueOf(src), "value", make(map[visit]struct{}), true)
+}
+
 // CopyMap validates and deeply copies an extra-parameter map while preserving
 // compatible concrete map, slice, array, scalar, and number types.
 func CopyMap(src map[string]any) (map[string]any, error) {
 	if src == nil {
 		return nil, nil
 	}
-	copied, err := copyValue(reflect.ValueOf(src), "extra_params", make(map[visit]struct{}))
+	copied, err := copyValue(reflect.ValueOf(src), "extra_params", make(map[visit]struct{}), false)
 	if err != nil {
 		return nil, err
 	}
@@ -35,7 +41,12 @@ func CopyMap(src map[string]any) (map[string]any, error) {
 	return out, nil
 }
 
-func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
+func copyValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
 	if !value.IsValid() {
 		return nil, nil
 	}
@@ -43,7 +54,7 @@ func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any,
 		if value.IsNil() {
 			return nil, nil
 		}
-		return copyValue(value.Elem(), path, seen)
+		return copyValue(value.Elem(), path, seen, allowEmptyMapKeys)
 	}
 	if !value.CanInterface() {
 		return nil, fmt.Errorf("%s: value cannot be copied", path)
@@ -88,22 +99,27 @@ func copyValue(value reflect.Value, path string, seen map[visit]struct{}) (any,
 		}
 		seen[current] = struct{}{}
 		defer delete(seen, current)
-		return copyValue(value.Elem(), path, seen)
+		return copyValue(value.Elem(), path, seen, allowEmptyMapKeys)
 	case reflect.Map:
-		return copyMapValue(value, path, seen)
+		return copyMapValue(value, path, seen, allowEmptyMapKeys)
 	case reflect.Slice:
 		if value.IsNil() {
 			return nil, nil
 		}
-		return copySequenceValue(value, path, seen)
+		return copySequenceValue(value, path, seen, allowEmptyMapKeys)
 	case reflect.Array:
-		return copySequenceValue(value, path, seen)
+		return copySequenceValue(value, path, seen, allowEmptyMapKeys)
 	default:
 		return nil, fmt.Errorf("%s: unsupported JSON value type %s", path, value.Type())
 	}
 }
 
-func copyMapValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
+func copyMapValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
 	if value.IsNil() {
 		return nil, nil
 	}
@@ -133,10 +149,10 @@ func copyMapValue(value reflect.Value, path string, seen map[visit]struct{}) (an
 	elementType := value.Type().Elem()
 	for _, key := range keys {
 		name := key.String()
-		if name == "" {
+		if name == "" && !allowEmptyMapKeys {
 			return nil, fmt.Errorf("%s: map key must not be empty", path)
 		}
-		copied, err := copyValue(value.MapIndex(key), path+"."+name, seen)
+		copied, err := copyValue(value.MapIndex(key), path+"."+name, seen, allowEmptyMapKeys)
 		if err != nil {
 			return nil, err
 		}
@@ -171,7 +187,12 @@ func copyMapValue(value reflect.Value, path string, seen map[visit]struct{}) (an
 	return out, nil
 }
 
-func copySequenceValue(value reflect.Value, path string, seen map[visit]struct{}) (any, error) {
+func copySequenceValue(
+	value reflect.Value,
+	path string,
+	seen map[visit]struct{},
+	allowEmptyMapKeys bool,
+) (any, error) {
 	var current visit
 	if value.Kind() == reflect.Slice {
 		current = visit{typ: value.Type(), ptr: value.Pointer()}
@@ -186,7 +207,12 @@ func copySequenceValue(value reflect.Value, path string, seen map[visit]struct{}
 	preserveType := true
 	elementType := value.Type().Elem()
 	for i := 0; i < value.Len(); i++ {
-		copied, err := copyValue(value.Index(i), fmt.Sprintf("%s[%d]", path, i), seen)
+		copied, err := copyValue(
+			value.Index(i),
+			fmt.Sprintf("%s[%d]", path, i),
+			seen,
+			allowEmptyMapKeys,
+		)
 		if err != nil {
 			return nil, err
 		}

internal/jsonvalue/jsonvalue_test.go

@@ -44,6 +44,21 @@ func TestCopyMapPreservesTypesAndIsolatesMutations(t *testing.T) {
 	}
 }
 
+func TestCopyAllowsEmptyObjectKeysAndIsolatesMutations(t *testing.T) {
+	nested := map[string]any{"": []any{"original"}}
+
+	copiedValue, err := jsonvalue.Copy(nested)
+	if err != nil {
+		t.Fatalf("copy value: %v", err)
+	}
+	nested[""].([]any)[0] = "changed"
+
+	copied := copiedValue.(map[string]any)
+	if got := copied[""].([]any)[0]; got != "original" {
+		t.Fatalf("copied value was not isolated: %v", got)
+	}
+}
+
 func TestCopyMapRejectsInvalidValues(t *testing.T) {
 	cyclicMap := map[string]any{}
 	cyclicMap["self"] = cyclicMap