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.

Reproduce the issue in a general GitHub Copilot CLI chat using open_canvas with canvasId "editor" and a valid path but no scope. Inspect list_canvas_capabilities and the open_canvas tool description to compare the declared schema with the documented inputs. Done means either the editor opens without scope or the required field and valid values are clearly surfaced.

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