fix(multiagent): remove no-op asyncio.gather in parallel execution

- Remove unnecessary asyncio.gather() after event loop completion
- Same issue as tool executor PR strands-agents#954
- By the time loop exits, all tasks have already completed
- Gather was waiting for already-finished tasks (no-op)
- All 154 tests passing

Author: Murat Kaan Meral

src/strands/multiagent/graph.py

@@ -625,10 +625,6 @@ async def stream_node_to_queue(node: GraphNode, node_index: int) -> None:
                 # Forward the event immediately
                 yield event
 
-        # Wait for all tasks to complete (should be immediate since they've all signaled completion)
-        if active_tasks:
-            await asyncio.gather(*active_tasks, return_exceptions=True)
-
     def _find_newly_ready_nodes(self, completed_batch: list["GraphNode"]) -> list["GraphNode"]:
         """Find nodes that became ready after the last execution."""
         newly_ready = []