fix: Pre-merge stateDelta before onUserMessageCallback (#1099)

The stateDelta was not merged into session state before onUserMessageCallback was invoked, causing plugins to see null values when reading caller-provided state entries.

This fix pre-merges stateDelta into the session's in-memory state before creating the InvocationContext. Since the session is already a copy from getSession(), this is safe and does not affect the persistence path which still happens via EventActions in appendNewMessageToSession.

Added test: onUserMessageCallback_withStateDelta_seesMergedState

Author: YuqiGuo105

core/src/main/java/com/google/adk/runner/Runner.java

@@ -448,6 +448,12 @@ protected Flowable<Event> runAsyncImpl(
               BaseAgent rootAgent = this.agent;
               String invocationId = InvocationContext.newInvocationContextId();
 
+              // Pre-merge stateDelta so onUserMessageCallback can access it.
+              // Safe: session is a copy; persistence still happens via appendNewMessageToSession.
+              if (stateDelta != null && !stateDelta.isEmpty()) {
+                stateDelta.forEach((key, value) -> session.state().put(key, value));
+              }
+
               // Create initial context
               InvocationContext initialContext =
                   newInvocationContextBuilder(session)

core/src/test/java/com/google/adk/runner/RunnerTest.java

@@ -877,6 +877,38 @@ public void beforeRunCallback_withStateDelta_seesMergedState() {
     assertThat(sessionInCallback.state()).containsEntry("number", 123);
   }
 
+  @Test
+  public void onUserMessageCallback_withStateDelta_seesMergedState() {
+    ArgumentCaptor<InvocationContext> contextCaptor =
+        ArgumentCaptor.forClass(InvocationContext.class);
+    when(plugin.onUserMessageCallback(contextCaptor.capture(), any())).thenReturn(Maybe.empty());
+
+    ImmutableMap<String, Object> stateDelta =
+        ImmutableMap.of("callback_key", "callback_value", "number", 123);
+
+    var unused =
+        runner
+            .runAsync(
+                "user",
+                session.id(),
+                createContent("test with state"),
+                RunConfig.builder().build(),
+                stateDelta)
+            .toList()
+            .blockingGet();
+
+    // Verify onUserMessageCallback was called
+    verify(plugin).onUserMessageCallback(any(), any());
+
+    // Verify the context passed to onUserMessageCallback has the merged state
+    InvocationContext capturedContext = contextCaptor.getValue();
+    Session sessionInCallback = capturedContext.session();
+
+    // Verify state delta was merged before onUserMessageCallback was invoked
+    assertThat(sessionInCallback.state()).containsEntry("callback_key", "callback_value");
+    assertThat(sessionInCallback.state()).containsEntry("number", 123);
+  }
+
   @Test
   public void runAsync_ensureEventsAreAppendedInOrder() throws Exception {
     Event event1 = TestUtils.createEvent("1");