feat: Add Graceful Plugin Shutdown to Runner

This change introduces a shutdown lifecycle hook for plugins. The `PluginManager` now has an `async def shutdown()` method that will call `await plugin.shutdown()` on any registered plugins that implement the method. This is called from `Runner.close()`, allowing plugins to perform cleanup tasks like flushing logs or closing connections when the runner instance is being closed. This improves the reliability of plugins that perform background operations.

PiperOrigin-RevId: 831037737

src/google/adk/plugins/bigquery_agent_analytics_plugin.py

@@ -455,7 +455,7 @@ class BigQueryAgentAnalyticsPlugin(BasePlugin):
     self._background_tasks.add(task)
     task.add_done_callback(self._background_tasks.discard)
 
-  async def shutdown(self):
+  async def close(self):
     """Flushes pending logs and closes client."""
     # 1. Wait for pending background logs (best effort, 2s timeout)
     if self._background_tasks: