Task 8.2.1: Implement open canvas in workspace method

[jaypaulb]

Implement a method in CanvusClient to open a canvas in a workspace using the POST /clients/:client_id/workspaces/:workspace_index/open-canvas endpoint. Accepts client_id, workspace_index, canvas_id, and server_id. Returns response as dictionary. Add tests and update documentation (EXAMPLES.md, Docs/Workspaces_API.md).