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/agent_servers.rs

@@ -18,6 +18,7 @@ use project::Project;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use std::{
+    any::Any,
     path::{Path, PathBuf},
     rc::Rc,
     sync::Arc,
@@ -40,6 +41,14 @@ pub trait AgentServer: Send {
         project: &Entity<Project>,
         cx: &mut App,
     ) -> Task<Result<Rc<dyn AgentConnection>>>;
+
+    fn into_any(self: Rc<Self>) -> Rc<dyn Any>;
+}
+
+impl dyn AgentServer {
+    pub fn downcast<T: 'static + AgentServer + Sized>(self: Rc<Self>) -> Option<Rc<T>> {
+        self.into_any().downcast().ok()
+    }
 }
 
 impl std::fmt::Debug for AgentServerCommand {

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 {

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)]