Update to ACP SDK v0.8.0 (#44063)

Uses the latest version of the SDK + schema crate. A bit painful because
we needed to move to `#[non_exhaustive]` on all of these structs/enums,
but will be much easier going forward.

Also, since we depend on unstable features, I am pinning the version so
we don't accidentally introduce compilation errors from other update
cycles.

Release Notes:

- N/A

crates/agent_servers/src/codex.rs

@@ -42,7 +42,7 @@ impl AgentServer for Codex {
 
         settings
             .as_ref()
-            .and_then(|s| s.default_mode.clone().map(|m| acp::SessionModeId(m.into())))
+            .and_then(|s| s.default_mode.clone().map(acp::SessionModeId::new))
     }
 
     fn set_default_mode(&self, mode_id: Option<acp::SessionModeId>, fs: Arc<dyn Fs>, cx: &mut App) {
@@ -63,7 +63,7 @@ impl AgentServer for Codex {
 
         settings
             .as_ref()
-            .and_then(|s| s.default_model.clone().map(|m| acp::ModelId(m.into())))
+            .and_then(|s| s.default_model.clone().map(acp::ModelId::new))
     }
 
     fn set_default_model(&self, model_id: Option<acp::ModelId>, fs: Arc<dyn Fs>, cx: &mut App) {