Prevent sending slash commands in CC threads (#36453)

Highlight them as errors in the editor, and add a leading space when
sending them so users don't hit the odd behavior when sending these
commands to the SDK.

Release Notes:

- N/A

crates/agent2/src/native_agent_server.rs

@@ -1,4 +1,4 @@
-use std::{path::Path, rc::Rc, sync::Arc};
+use std::{any::Any, path::Path, rc::Rc, sync::Arc};
 
 use agent_servers::AgentServer;
 use anyhow::Result;
@@ -66,4 +66,8 @@ impl AgentServer for NativeAgentServer {
             Ok(Rc::new(connection) as Rc<dyn acp_thread::AgentConnection>)
         })
     }
+
+    fn into_any(self: Rc<Self>) -> Rc<dyn Any> {
+        self
+    }
 }