Extension canvas.open emits malformed opened event and does not reliably focus #4112

Problem

A user-scoped Copilot CLI extension can register and manually render a canvas, but calling the documented low-level session.rpc.canvas.open route on resume does not reliably make the canvas visible and emits a malformed event warning in the extension process.

Copilot CLI version: 1.0.71-0 on macOS arm64.

Reproduction

1. Register a canvas through joinSession({ canvases: [...], requestCanvasRenderer: true }).
2. After detecting a resumed session, resolve the canvas through session.rpc.canvas.list().
3. Call session.rpc.canvas.open with the exact returned provider ID:

await session.rpc.canvas.open({
  extensionId: declared.extensionId,
  canvasId: declared.canvasId,
  instanceId: "stable-instance",
  input: { key: "example" },
});

A complete implementation and focused tests are available in the Session Briefing adapter and its test.

Actual behavior

• The canvas.open promise resolves; the extension's bounded failure diagnostic does not run.
• The extension process logs failed to deserialize session.canvas.opened payload.
• The resumed CLI does not reliably show/focus the canvas.
• The SDK rejects the event because the payload does not satisfy OpenCanvasInstance (provider identity, instance/canvas IDs, reopen, and availability).

Expected behavior

The returned/opened snapshot and session.canvas.opened event satisfy the bundled SDK schema, and reopening a stable instance visibly focuses the canvas.

Additional evidence

• The same canvas renders successfully when opened through the host's open_canvas tool with the explicit provider ID.
• Omitting extensionId and passing the exact ID returned by canvas.list() both produce the malformed-event warning.
• The canvas ID is unique in the session.

A small diagnostic option to log or expose the rejected event shape would also make this class of failure much easier to debug without instrumenting the SDK.