feat(agent): support JSON object input on begin node (#16685)

### Summary

Add object as a begin-node parameter type with JSON editor UI, webhook
schema support, and backend parsing in UserFillUp.

Co-authored-by: Cursor <cursoragent@cursor.com>

web/src/locales/en.ts

@@ -2719,6 +2719,7 @@ This process aggregates variables from multiple branches into a single variable
       file: 'File upload',
       integer: 'Number',
       boolean: 'Boolean',
+      object: 'JSON object',
 
       logTimeline: {
         begin: 'Ready to begin',

web/src/locales/zh.ts

@@ -1781,6 +1781,7 @@ NER：使用 spaCy NER 和基于规则的关键词提取来抽取实体和关系
       file: '文件',
       integer: '数字',
       boolean: '布尔值',
+      object: 'JSON 对象',
       name: '名称',
       singleLineText: '单行文本',
       variableSettings: '变量设置',