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/agent_servers/src/gemini.rs

@@ -1,5 +1,5 @@
-use std::path::Path;
 use std::rc::Rc;
+use std::{any::Any, path::Path};
 
 use crate::{AgentServer, AgentServerCommand};
 use acp_thread::{AgentConnection, LoadError};
@@ -86,6 +86,10 @@ impl AgentServer for Gemini {
             result
         })
     }
+
+    fn into_any(self: Rc<Self>) -> Rc<dyn Any> {
+        self
+    }
 }
 
 #[cfg(test)]