Use selected model when starting Copilot coding agent task (#4820)

When setting the session type to "Cloud", VS Code correctly lists models available for Copilot coding agent.

However, the selected model is not honored because it is sent in the `model_name` parameter instead of the expected `model` parameter.

This switches to using the expected `model` parameter, as expected by the `RemoteAgentJobPayload` type and the underlying API.