Refactor: remove unused variable and optimize redundant computation

- Removed unused variable `all_routes_node_ids` to reduce code clutter.
- Moved `set(routes_node_ids.keys())` outside the loop to avoid repeated computation.

api/core/workflow/graph_engine/entities/graph.py

@@ -639,11 +639,8 @@ class Graph(BaseModel):
         if start_node_id not in reverse_edge_mapping:
             return False
 
-        all_routes_node_ids = set()
         parallel_start_node_ids: dict[str, list[str]] = {}
-        for branch_node_id, node_ids in routes_node_ids.items():
-            all_routes_node_ids.update(node_ids)
-
+        for branch_node_id in routes_node_ids.items():
             if branch_node_id in reverse_edge_mapping:
                 for graph_edge in reverse_edge_mapping[branch_node_id]:
                     if graph_edge.source_node_id not in parallel_start_node_ids:
@@ -651,8 +648,9 @@ class Graph(BaseModel):
 
                     parallel_start_node_ids[graph_edge.source_node_id].append(branch_node_id)
 
+        expected_branch_set = set(routes_node_ids.keys())
         for _, branch_node_ids in parallel_start_node_ids.items():
-            if set(branch_node_ids) == set(routes_node_ids.keys()):
+            if set(branch_node_ids) == expected_branch_set:
                 return True
 
         return False