refactor(api): Use UPPER_SNAKE_CASE name for enum members

Change the enum member names for `DraftVariableType`.

api/models/enums.py

@@ -18,6 +18,6 @@ class WorkflowRunTriggeredFrom(StrEnum):
 
 class DraftVariableType(StrEnum):
     # node means that the correspond variable
-    node = "node"
-    sys = "sys"
-    conversation = "conversation"
+    NODE = "node"
+    SYS = "sys"
+    CONVERSATION = "conversation"

api/models/workflow.py

@@ -918,19 +918,19 @@ class WorkflowDraftVariable(Base):
         self.value_type = value.value_type
 
     def get_node_id(self) -> str | None:
-        if self.get_variable_type() == DraftVariableType.node:
+        if self.get_variable_type() == DraftVariableType.NODE:
             return self.node_id
         else:
             return None
 
     def get_variable_type(self) -> DraftVariableType:
         match self.node_id:
-            case DraftVariableType.conversation:
-                return DraftVariableType.conversation
-            case DraftVariableType.sys:
-                return DraftVariableType.sys
+            case DraftVariableType.CONVERSATION:
+                return DraftVariableType.CONVERSATION
+            case DraftVariableType.SYS:
+                return DraftVariableType.SYS
             case _:
-                return DraftVariableType.node
+                return DraftVariableType.NODE
 
     @classmethod
     def _create(