Revert to synchronous to fix tests

Author: timja

src/main/java/io/jenkins/plugins/pipelinegraphview/livestate/LiveGraphPopulator.java

@@ -13,14 +13,16 @@
  * feeds it to the corresponding {@link LiveGraphState}. The downstream {@code PipelineGraphApi}
  * path reads a snapshot of that state instead of walking the whole execution each time.
  *
- * <p>We use the async {@link GraphListener} rather than {@code GraphListener.Synchronous}:
- * HTTP readers poll every few seconds, so we only need eventual consistency, and keeping
- * the listener off the CPS VM thread avoids any risk of blocking execution. Every code path
- * is still wrapped in try/catch and poisons the state on failure so a bug here can never
- * disrupt a build.
+ * <p>We use {@link GraphListener.Synchronous} rather than the async variant because callers
+ * expect "once a node is a head, the next API read reflects it" — async delivery creates a
+ * lag window where the snapshot is behind the execution, which breaks tests that check state
+ * at precise trigger points and would surprise anyone hitting the REST API after an event.
+ * The work done under the monitor is trivial ({@code ArrayList}/{@code HashSet} additions),
+ * so the CPS VM thread is not meaningfully blocked. Every code path is still wrapped in
+ * try/catch and poisons the state on failure so a bug here can never disrupt a build.
  */
 @Extension
-public class LiveGraphPopulator implements GraphListener {
+public class LiveGraphPopulator implements GraphListener.Synchronous {
 
     private static final Logger logger = LoggerFactory.getLogger(LiveGraphPopulator.class);