open_canvas: Invalid input error when 'scope' field is missing from editor canvas

### Short summary

`open_canvas` for the `editor` canvas type returns an invalid input error when the `scope` field is not provided, even though it is not documented as required.

### Affected version or release

1.0.64-0

### Installation context

GitHub Copilot CLI chat session (general chat, not tied to a project session)

### What happened?

When calling `open_canvas` with `canvasId: "editor"` and a valid `path` in the input, the tool returned:

```
dDe: Invalid input for canvas "editor" open input: (root): "scope" is a required property. Inspect the canvas declaration for the expected input schema.
```

The canvas did not open. No `scope` field is listed or described in the canvas documentation or tool description.

### Steps to reproduce

1. From a general chat session (not a project session), call `open_canvas` with:
```json
{
"canvasId": "editor",
"instanceId": "my-canvas-instance",
"input": {
"path": "C:\\Users\\\\.copilot\\session-state\\\\files\\example.md"
}
}
```
2. Observe the error response.

### Expected behavior

The editor canvas should open with the specified file, or the error message should clearly indicate that `scope` is a required field and document valid values (e.g., `user`, `project`, `session`).

### Additional context

- The `list_canvas_capabilities` tool would reveal the schema, but the error message itself gives no hint.
- The `open_canvas` tool description does not mention a required `scope` field for the `editor` canvas.
- Surfacing required fields in the error message or tool description would prevent this confusion.

Reproduisez le problème dans un chat GitHub Copilot CLI général en utilisant open_canvas avec canvasId "editor" et un chemin valide, mais sans scope. Inspectez list_canvas_capabilities et la description de l’outil open_canvas afin de comparer le schéma déclaré aux entrées documentées. C’est terminé si l’éditeur s’ouvre sans scope ou si le champ requis et les valeurs valides sont clairement affichés.

Rédigé par le modèle d'indexation à partir du texte de l'issue.