ai onboarding: Add telemetry event capturing (#34960)

Release Notes:

- N/A

Co-authored-by: Katie Geer <katie@zed.dev>
Co-authored-by: Joseph T. Lyons <JosephTLyons@gmail.com>

crates/agent_ui/src/message_editor.rs

@@ -910,6 +910,10 @@ impl MessageEditor {
                                                         .on_click({
                                                             let focus_handle = focus_handle.clone();
                                                             move |_event, window, cx| {
+                                                                telemetry::event!(
+                                                                    "Agent Message Sent",
+                                                                    agent = "zed",
+                                                                );
                                                                 focus_handle.dispatch_action(
                                                                     &Chat, window, cx,
                                                                 );