Plugin canvas installed during a session is unavailable until a new session

When a plugin contributing a canvas is installed during an active session, the canvas remains unavailable until a new session is started.

`extensions_reload` only rescans user, project, and session extension folders—not plugin-contributed extensions—so reloading does not make the newly installed canvas available.

### Steps to reproduce

1. Start a Copilot App session.
2. Install a plugin that contributes a canvas.
3. Reload extensions or ask Copilot to open the canvas.
4. Observe that the canvas is unavailable.
5. Start a new session; the canvas becomes available.

### Expected behavior

Automatically refresh or restart the session after plugin installation, or clearly inform the user that a new session is required.

### Environment

- Copilot App: `1.0.79-9`
- OS: Windows

首先跟踪插件安装流程和 extensions_reload 入口点，重点关注如何处理插件提供的扩展和会话刷新。使用一个为 canvas 提供功能的插件重现所列步骤；完成的标准是 canvas 在活动会话期间可用，或者应用明确告知用户需要新会话。

由索引模型根据 Issue 内容生成。