fix(api): defer streaming response until referenced variables are updated (#30832)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

api/core/workflow/nodes/variable_assigner/v1/node.py

@@ -33,6 +33,15 @@ class VariableAssignerNode(Node[VariableAssignerData]):
             graph_runtime_state=graph_runtime_state,
         )
 
+    def blocks_variable_output(self, variable_selectors: set[tuple[str, ...]]) -> bool:
+        """
+        Check if this Variable Assigner node blocks the output of specific variables.
+
+        Returns True if this node updates any of the requested conversation variables.
+        """
+        assigned_selector = tuple(self.node_data.assigned_variable_selector)
+        return assigned_selector in variable_selectors
+
     @classmethod
     def version(cls) -> str:
         return "1"