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>

agent/component/fillup.py

@@ -76,7 +76,13 @@ class UserFillUp(ComponentBase):
             return FileService.get_files(files, layout_recognize=layout_recognize)
 
         if isinstance(value, dict):
-            return value.get("value")
+            raw = value.get("value")
+            if value.get("type") == "object" and isinstance(raw, str) and raw.strip():
+                try:
+                    return json.loads(raw)
+                except Exception:
+                    return raw
+            return raw
 
         return value