Replace HTML injection pattern with AJAX command pipeline

Problem/Motivation

AI Plus delivers UI side effects (page refresh, open-link navigation, workspace toolbar updates) from server-side AI tool execution to the browser. The current mechanism injects hidden HTML trigger spans into the chat response text and uses a MutationObserver on DeepChat's shadow DOM to detect them.

This approach has several problems:

• Fragile timing — the observer must attach before the shadow DOM renders, and DeepChat's initialization order is not guaranteed
• Single-fire — only one trigger span per response type, no way to queue multiple commands of the same kind
• Untestable — behavior depends on DOM mutation timing in a shadow root
• Split architecture — polling mode (interval-based change-check) and instant mode (trigger span detection) are two separate code paths solving the same problem differently, doubling maintenance surface
• State API pollution — uses Drupal State to communicate between the AI tool request and the subsequent polling/trigger request, leaking per-user ephemeral data into a global store

Proposed resolution

Replace the HTML injection pattern with Drupal's native AJAX command system, piggybacking on the DeepChat JSON response:

1. UiCommandQueue — shared in-memory queue where event subscribers add CommandInterface objects during the request
2. DeepChatResponseListener — a kernel.response listener on the ai_chatbot.api route that flushes the queue and injects rendered commands as a drupal_commands array into the JSON response
3. ai-plus-command-processor.js — wraps DeepChat's responseInterceptor to extract drupal_commands and execute them via Drupal.ajax().success()