ACP over MCP server impl (#35196)

Release Notes:

- N/A

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/agent_ui/src/agent_ui.rs

@@ -150,6 +150,7 @@ enum ExternalAgent {
     #[default]
     Gemini,
     ClaudeCode,
+    Codex,
 }
 
 impl ExternalAgent {
@@ -157,6 +158,7 @@ impl ExternalAgent {
         match self {
             ExternalAgent::Gemini => Rc::new(agent_servers::Gemini),
             ExternalAgent::ClaudeCode => Rc::new(agent_servers::ClaudeCode),
+            ExternalAgent::Codex => Rc::new(agent_servers::Codex),
         }
     }
 }