Lock system.heartbeat wire name with a contract test

The extension emits the literal event string "system.heartbeat";
assert the daemon-side serde name serialises to the same value and
round-trips from the exact frame shape the extension sends, so a
future rename cannot silently break keepalive and liveness.

Co-authored-by: Cursor <cursoragent@cursor.com>

crates/bsk-protocol/src/frame.rs

@@ -238,6 +238,24 @@ impl<'de> Visitor<'de> for FrameVisitor {
 mod tests {
     use super::*;
 
+    #[test]
+    fn system_heartbeat_serialises_as_dotted_name() {
+        // The extension hardcodes the literal string "system.heartbeat"
+        // when it emits the keepalive event; this locks the daemon-side
+        // serde name to the same wire value so a rename cannot silently
+        // break liveness/keepalive.
+        let v = serde_json::to_value(EventKind::SystemHeartbeat).unwrap();
+        assert_eq!(v, serde_json::json!("system.heartbeat"));
+    }
+
+    #[test]
+    fn system_heartbeat_event_frame_round_trips_from_extension_shape() {
+        // Mirrors exactly what the extension sends: { event, payload: {} }.
+        let wire = serde_json::json!({ "event": "system.heartbeat", "payload": {} });
+        let frame: EventFrame = serde_json::from_value(wire).unwrap();
+        assert_eq!(frame.event, EventKind::SystemHeartbeat);
+    }
+
     #[test]
     fn session_user_interrupt_serialises_as_snake_case() {
         let v = serde_json::to_value(EventKind::SessionUserInterrupt).unwrap();