acp: Fix /logout for agents that support it (#40248)

We were clearing the message editor too early. We only want to clear the
message editor if we are going to short circuit and return early before
submitting.
Otherwise, the agents that can handle this themselves won't have the
ability to do so.

Release Notes:

- acp: Fix /logout not working for some agents

crates/agent_ui/src/acp/thread_view.rs

@@ -1045,9 +1045,6 @@ impl AcpThreadView {
                 return;
             };
 
-            self.message_editor
-                .update(cx, |editor, cx| editor.clear(window, cx));
-
             let connection = thread.read(cx).connection().clone();
             let can_login = !connection.auth_methods().is_empty() || self.login.is_some();
             // Does the agent have a specific logout command? Prefer that in case they need to reset internal state.
@@ -1058,6 +1055,9 @@ impl AcpThreadView {
                     .iter()
                     .any(|command| command.name == "logout");
             if can_login && !logout_supported {
+                self.message_editor
+                    .update(cx, |editor, cx| editor.clear(window, cx));
+
                 let this = cx.weak_entity();
                 let agent = self.agent.clone();
                 window.defer(cx, |window, cx| {