Refactor message sending architecture and implement legacy driver support

- Removed UniversalMessageSender from group_generator.py and private_generator.py.
- Updated PlatformIOManager to manage legacy send drivers and ensure send pipeline readiness.
- Enhanced LegacyPlatformDriver to utilize prepared messages for sending.
- Refactored send_service to unify message sending logic and integrate with Platform IO.
- Added regression tests for Platform IO legacy driver and send service functionality.

src/plugin_runtime/integration.py

@@ -157,7 +157,7 @@ class PluginRuntimeManager(
         started_supervisors: List[PluginSupervisor] = []
         try:
             platform_io_manager.set_inbound_dispatcher(self._dispatch_platform_inbound)
-            await platform_io_manager.start()
+            await platform_io_manager.ensure_send_pipeline_ready()
 
             if self._builtin_supervisor:
                 await self._builtin_supervisor.start()