Fix Vertex session user IDs

Author: Roland Kakonyi

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

@@ -148,7 +148,7 @@ private ListSessionsResponse parseListSessionsResponse(
       Session session =
           Session.builder(sessionId)
               .appName(appName)
-              .userId(userId)
+              .userId((String) apiSession.get("userId"))
               .state(
                   apiSession.get("sessionState") == null
                       ? new ConcurrentHashMap<>()

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

@@ -279,6 +279,34 @@ public void listSessions_success() {
     assertThat(sessionsList).hasSize(2);
     ImmutableList<String> ids = sessionsList.stream().map(Session::id).collect(toImmutableList());
     assertThat(ids).containsExactly("1", "2");
+    ImmutableList<String> userIds =
+        sessionsList.stream().map(Session::userId).collect(toImmutableList());
+    assertThat(userIds).containsExactly("user", "user");
+  }
+
+  @Test
+  public void listSessions_usesResponseUserId() throws Exception {
+    when(mockApiClient.request(
+            "GET", "reasoningEngines/123/sessions?filter=user_id%3D%22user1%22", ""))
+        .thenAnswer(
+            new MockApiAnswer(
+                """
+                {
+                  "sessions": [
+                    {
+                      "name": "projects/test-project/locations/test-location/reasoningEngines/123/sessions/3",
+                      "userId": "user2",
+                      "updateTime": "2024-12-14T12:12:12.123456Z"
+                    }
+                  ]
+                }\
+                """));
+
+    ListSessionsResponse sessions =
+        vertexAiSessionService.listSessions("123", "user1").blockingGet();
+
+    assertThat(sessions.sessions()).hasSize(1);
+    assertThat(sessions.sessions().get(0).userId()).isEqualTo("user2");
   }
 
   @Test