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/claude.rs

@@ -65,6 +65,10 @@ impl AgentServer for ClaudeCode {
 
         Task::ready(Ok(Rc::new(connection) as _))
     }
+
+    fn into_any(self: Rc<Self>) -> Rc<dyn Any> {
+        self
+    }
 }
 
 struct ClaudeAgentConnection {