feat(core): support compose a doc tool (#13013)

#### PR Dependency Tree


* **PR #13013** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Introduced a document composition tool for AI chat, allowing users to
generate, preview, and save structured markdown documents directly from
chat interactions.
* Added an artifact preview panel for enhanced document previews within
the chat interface.
* Enabled dynamic content rendering in the chat panel's right section
for richer user experiences.

* **Improvements**
  * Sidebar maximum width increased for greater workspace flexibility.
* Enhanced chat message and split view styling for improved layout and
usability.

* **Bug Fixes**
  * None.

* **Other**
* Registered new custom elements for AI tools and artifact preview
functionality.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/tools/index.ts

@@ -1,3 +1,4 @@
+export * from './doc-compose';
 export * from './doc-edit';
 export * from './doc-keyword-search';
 export * from './doc-read';