Add previewArgs support for template functions and enhance validation logic for form inputs

plugins/template-function-timestamp/src/index.ts

@@ -81,12 +81,14 @@ export const plugin: PluginDefinition = {
       name: 'timestamp.format',
       description: 'Format a date using a dayjs-compatible format string',
       args: [dateArg, formatArg],
+      previewArgs: [formatArg.name],
       onRender: async (_ctx, args) => formatDatetime(args.values),
     },
     {
       name: 'timestamp.offset',
       description: 'Get the offset of a date based on an expression',
       args: [dateArg, expressionArg],
+      previewArgs: [expressionArg.name],
       onRender: async (_ctx, args) => calculateDatetime(args.values),
     },
   ],