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

Start by tracing the plugin installation flow and the extensions_reload entry point, focusing on how plugin-contributed extensions and session refreshes are handled. Reproduce the listed steps with a canvas-contributing plugin; done means the canvas becomes available during the active session or the app clearly tells the user that a new session is required.

索引モデルが issue の本文から書いたものです。