Support session/set_model in ACP

[jhchee]

Describe the feature or problem you'd like to solve

ACP supports model switching through session/set_model, however when this method is invoked, Copilot ACP didn't switch underlying model.

Proposed solution

Support session/set_model in Copilot ACP implement so user can switch the underlying model.

Example prompts or workflows

await conn.set_session_model("gpt-5-mini", session_id)

switches the underlying model the ACP session interacts with

Additional context

No response