fix: remove is a state mutation operation and should also be captured in the delta

PiperOrigin-RevId: 785569259

Author: google-genai-bot

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

@@ -119,12 +119,19 @@ public void putAll(Map<? extends String, ? extends Object> m) {
 
   @Override
   public Object remove(Object key) {
+    if (state.containsKey(key)) {
+      delta.put((String) key, null);
+    }
     return state.remove(key);
   }
 
   @Override
   public boolean remove(Object key, Object value) {
-    return state.remove(key, value);
+    boolean removed = state.remove(key, value);
+    if (removed) {
+      delta.put((String) key, null);
+    }
+    return removed;
   }
 
   @Override