fix(platform): websocket send_message not delivering to webchat frontend (#2039)

- Include websocket_proxy_bot in get_bot_by_uuid lookup so plugins can
  find it by uuid
- Rewrite send_message to broadcast directly via ws_connection_manager
  using the correct pipeline_uuid instead of misusing target_id
- Save messages to session history with unique IDs so they persist
  across page reloads and don't overwrite each other

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/langbot/pkg/platform/botmgr.py

@@ -282,6 +282,8 @@ class PlatformManager:
         return runtime_bot
 
     async def get_bot_by_uuid(self, bot_uuid: str) -> RuntimeBot | None:
+        if self.websocket_proxy_bot and self.websocket_proxy_bot.bot_entity.uuid == bot_uuid:
+            return self.websocket_proxy_bot
         for bot in self.bots:
             if bot.bot_entity.uuid == bot_uuid:
                 return bot