fix(expression-eval): fix variable extraction issue in Jinja2 templates

- Resolve the bug where variables inside Jinja2 template expressions were not correctly extracted.
- Ensure expressions containing {{ ... }} are parsed reliably.

api/app/core/workflow/expression_evaluator.py

@@ -5,6 +5,7 @@
 """
 
 import logging
+import re
 from typing import Any
 
 from simpleeval import simple_eval, NameNotDefined, InvalidExpression
@@ -59,8 +60,9 @@ class ExpressionEvaluator:
         """
         # 移除 Jinja2 模板语法的花括号（如果存在）
         expression = expression.strip()
-        if expression.startswith("{{") and expression.endswith("}}"):
+        # "{{system.message}} == {{ user.messge }}" -> "system.message == user.message"
-            expression = expression[2:-2].strip()
+        pattern = r"\{\{\s*(.*?)\s*\}\}"
+        expression = re.sub(pattern, r"\1", expression).strip()
 
         # 构建命名空间上下文
         context = {