Canvas unavailable after extensions_reload until second reload

## Summary

After `extensions_reload`, a canvas extension can be listed as ready, but `open_canvas` fails with `CanvasRuntimeError: No canvas "inbox-zero" is registered`. Running `extensions_reload` a second time restores `open_canvas`.

## Repro steps

1. Start a canvas extension and open an instance, e.g. `inbox-zero-live`.
2. Run `extensions_reload`.
3. Confirm `extensions_manage({ operation: "list" })` shows the extension as ready.
4. Run `open_canvas({ canvasId: "inbox-zero", instanceId: "inbox-zero-live" })`.

## Observed

- `open_canvas` fails with `No canvas "inbox-zero" is registered`.
- `invoke_canvas_action(...)` fails with `Canvas instance "inbox-zero-live" is not open`.
- Running `extensions_reload` again makes `open_canvas` succeed.
- The previously open canvas can remain blank or stale until recovery.

## Expected

After one successful `extensions_reload`, ready canvas extensions should be immediately openable, and previously open canvas instances should recover without an additional reload.

## Environment

- Copilot CLI 1.0.63-0, Windows
- User-scope canvas extension: `~/.copilot/extensions/inbox-zero/`
- Canvas uses a loopback HTTP server

首先，使用 inbox-zero 扩展通过 extensions_reload、extensions_manage、open_canvas 和 invoke_canvas_action 重现该序列。跟踪 reload 和 canvas 实例恢复的生命周期，然后验证一次 reload 就能让已就绪的扩展可打开，并在不进行第二次 reload 的情况下恢复之前已打开的实例。

由索引模型根据 Issue 内容生成。