chore: use from __future__ import annotations (#30254)

Co-authored-by: Dev <dev@Devs-MacBook-Pro-4.local>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Asuka Minato <i@asukaminato.eu.org>
Co-authored-by: crazywoola <100913391+crazywoola@users.noreply.github.com>

api/core/workflow/runtime/variable_pool.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import re
 from collections import defaultdict
 from collections.abc import Mapping, Sequence
@@ -267,6 +269,6 @@ class VariablePool(BaseModel):
             self.add(selector, value)
 
     @classmethod
-    def empty(cls) -> "VariablePool":
+    def empty(cls) -> VariablePool:
         """Create an empty variable pool."""
         return cls(system_variables=SystemVariable.empty())