chore(api): ensure `variable_dictionary` is always a defaultdict

Even after serialization / deserialization roundtrip

api/core/workflow/entities/variable_pool.py

@@ -1,7 +1,7 @@
 import re
 from collections import defaultdict
 from collections.abc import Mapping, Sequence
-from typing import Any, Union, cast
+from typing import Annotated, Any, Union, cast
 
 from pydantic import BaseModel, Field
 
@@ -24,7 +24,7 @@ class VariablePool(BaseModel):
     # The first element of the selector is the node id, it's the first-level key in the dictionary.
     # Other elements of the selector are the keys in the second-level dictionary. To get the key, we hash the
     # elements of the selector except the first one.
-    variable_dictionary: dict[str, dict[int, VariableUnion]] = Field(
+    variable_dictionary: defaultdict[str, Annotated[dict[int, VariableUnion], Field(default_factory=dict)]] = Field(
         description="Variables mapping",
         default=defaultdict(dict),
     )
@@ -85,9 +85,6 @@ class VariablePool(BaseModel):
             variable = variable_factory.segment_to_variable(segment=segment, selector=selector)
 
         key, hash_key = self._selector_to_keys(selector)
-        # Ensure the first-level key exists in the dictionary
-        if key not in self.variable_dictionary:
-            self.variable_dictionary[key] = {}
         # Based on the definition of `VariableUnion`,
         # `list[Variable]` can be safely used as `list[VariableUnion]` since they are compatible.
         self.variable_dictionary[key][hash_key] = cast(VariableUnion, variable)
@@ -190,10 +187,3 @@ class VariablePool(BaseModel):
     def empty(cls) -> "VariablePool":
         """Create an empty variable pool."""
         return cls(system_variables=SystemVariable.empty())
-
-    def dumps(self) -> str:
-        return self.model_dump_json()
-
-    @classmethod
-    def loads(cls, json_data: str) -> "VariablePool":
-        return VariablePool.model_validate_json(json_data)