Require precipitation timing in generated text

internal/generatedtext/json.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"strings"
 )
 
 func decodeGeneratedText[T any](data []byte, name string) (T, error) {
@@ -24,6 +25,21 @@ func decodeGeneratedText[T any](data []byte, name string) (T, error) {
 	return value, fmt.Errorf("decode %s generated text: multiple JSON values", name)
 }
 
+func requireGeneratedTextStringField(data []byte, name, field string) error {
+	var fields map[string]json.RawMessage
+	if err := json.Unmarshal(data, &fields); err != nil {
+		return fmt.Errorf("decode %s generated text: %w", name, err)
+	}
+	raw, ok := fields[field]
+	if !ok {
+		return fmt.Errorf("%s generated text %s is required", name, strings.ReplaceAll(field, "_", " "))
+	}
+	if bytes.Equal(bytes.TrimSpace(raw), []byte("null")) {
+		return fmt.Errorf("%s generated text %s must be a string", name, strings.ReplaceAll(field, "_", " "))
+	}
+	return nil
+}
+
 func normalizeGeneratedText[T any](value T, name string) ([]byte, error) {
 	normalized, err := json.Marshal(value)
 	if err != nil {