* refactor(server): validate ACP mode/model selection and subscribe to live state
Match Zed's selection validity model: resolve the requested mode/model
against availableModes/configOptions before issuing the ACP RPC, log a
warning instead of throwing on unknown values, and adopt the
currentValue echoed back in setSessionConfigOption responses. Subscribe
to current_mode_update / config_option_update so AgentManager re-emits
state when the agent changes selection unprompted.
Also drops the Copilot Autopilot auto-approve shim to mirror Zed's pure
pass-through requestPermission path.
* refactor(server): emit ACP session config drift via namespaced events
Replace the parallel subscribeToSessionState callback channel with three
payload-bearing variants on the existing AgentStreamEvent union
(mode_changed, model_changed, thinking_option_changed). Manager updates
its record from the event payload and emits state — no callbacks back
into the session. Variants suppressed before websocket dispatch so the
wire schema is unchanged.
