Extract timestamp as double for InMemorySessionService events

InMemorySessionService sets a Session's last modified time based on when
the last appended event's timestamp. The timestamp in an event is
recorded in millis while the Session's timestamp is an Instant. During
the transformation, Events perform this converstion using division.
Before this change, the timestamp was truncated to the second, yet the
code was trying to extract nanos which were always 0. This fixes that
bug with a simple type change. I've also added a test to prevent
regressions.

Author: tlanclos

core/src/main/java/com/google/adk/sessions/InMemorySessionService.java

@@ -154,19 +154,13 @@ public Maybe<Session> getSession(
     if (config.numRecentEvents().isEmpty() && config.afterTimestamp().isPresent()) {
       Instant threshold = config.afterTimestamp().get();
 
-      eventsInCopy.removeIf(
-          event -> getEventTimestampEpochSeconds(event) < threshold.getEpochSecond());
+      eventsInCopy.removeIf(event -> getInstantFromEvent(event).isBefore(threshold));
     }
 
     // Merge state into the potentially filtered copy and return
     return Maybe.just(mergeWithGlobalState(appName, userId, sessionCopy));
   }
 
-  // Helper to get event timestamp as epoch seconds
-  private long getEventTimestampEpochSeconds(Event event) {
-    return event.timestamp() / 1000L;
-  }
-
   @Override
   public Single<ListSessionsResponse> listSessions(String appName, String userId) {
     Objects.requireNonNull(appName, "appName cannot be null");
@@ -294,10 +288,7 @@ public Single<Event> appendEvent(Session session, Event event) {
   /** Converts an event's timestamp to an Instant. Adapt based on actual Event structure. */
   // TODO: have Event.timestamp() return Instant directly
   private Instant getInstantFromEvent(Event event) {
-    double epochSeconds = getEventTimestampEpochSeconds(event);
-    long seconds = (long) epochSeconds;
-    long nanos = (long) ((epochSeconds % 1.0) * 1_000_000_000L);
-    return Instant.ofEpochSecond(seconds, nanos);
+    return Instant.ofEpochMilli(event.timestamp());
   }
 
   /**

core/src/test/java/com/google/adk/sessions/InMemorySessionServiceTest.java

@@ -20,6 +20,7 @@
 import com.google.adk.events.Event;
 import com.google.adk.events.EventActions;
 import io.reactivex.rxjava3.core.Single;
+import java.time.Instant;
 import java.util.HashMap;
 import java.util.Optional;
 import java.util.concurrent.ConcurrentHashMap;
@@ -214,6 +215,24 @@ public void appendEvent_removesState() {
     assertThat(retrievedSessionRemove.state()).doesNotContainKey("temp:tempKey");
   }
 
+  @Test
+  public void appendEvent_updatesSessionTimestampWithFractionalSeconds() {
+    InMemorySessionService sessionService = new InMemorySessionService();
+    Session session =
+        sessionService.createSession("app", "user", new HashMap<>(), "session1").blockingGet();
+
+    // Add an event with a timestamp that contains a fractional second
+    Event eventAdd = Event.builder().timestamp(5500).build();
+    var unused = sessionService.appendEvent(session, eventAdd).blockingGet();
+
+    // Verify the last modified timestamp contains a fractional second
+    Session retrievedSession =
+        sessionService
+            .getSession(session.appName(), session.userId(), session.id(), Optional.empty())
+            .blockingGet();
+    assertThat(retrievedSession.lastUpdateTime()).isEqualTo(Instant.ofEpochSecond(5, 500000000L));
+  }
+
   @Test
   public void sequentialAgents_shareTempState() {
     InMemorySessionService sessionService = new InMemorySessionService();