fix(acp): forward Claude Code model picker selection to the adapter

[kojiromike]

Problem

The Claude Code (ACP) provider's model picker was display-only. goose read the adapter-advertised Model config option to populate the dropdown, but whatever you selected never reached claude-agent-acp — it kept using whatever its own config dictated (env / ~/.claude/settings.json / /model). The UI showed one model while a different one served the session.

Root cause

claude_acp.rs built its AcpProviderConfig with model_config_option_id: None. The generic ACP provider only pushes a model choice to the agent (via session/set_config_option) when that id is Some; with None, apply_model_if_changed short-circuits and sends nothing.

Fix

I verified against the installed adapter (@agentclientprotocol/claude-agent-acp) that it advertises the model as a select config option with id "model" (MODEL_CONFIG_ID), and its setSessionConfigOption handler applies the value via query.setModel(...) (with a resolveModelPreference fallback for non-exact values). So the adapter fully supports model changes over ACP.

Wire the picker's selection through by setting model_config_option_id: Some("model"), mirroring the Copilot provider — the only one that previously wired this up.

Testing

cargo check -p goose passes. The apply_model_if_changed behavior is already covered by generic tests in acp/provider.rs; this is a one-line config change with no new branching to test (Copilot's identical config is likewise untested at the provider level).

Fixes #10669

[samrusani]

Verified against the current claude-agent-acp adapter: it advertises the model config option as model, and setSessionConfigOption applies it through query.setModel. Goose's existing apply_model_if_changed_sends_set_config_option_on_change regression passes with this wiring, and the provider now activates that tested path. I also confirmed the patch is limited to the Claude ACP provider and the applicable CI is green. LGTM.