fix(event): keep message_id in additional fields

Event.messageId from #278 was a typed Jackson field, so a top-level
message_id on message.*, reminder.* and poll.* webhooks moved out of
getAdditionalFields() and getAdditionalFields().get("message_id")
returned null after upgrading.

getMessageId()/setMessageId() now read and write the additional-fields
map instead, so parsing and serialization match 1.50.0 and both
accessors return the ID on every event.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: nijeesh-stream

src/main/java/io/getstream/chat/java/models/Event.java

@@ -117,12 +117,35 @@ public class Event {
   @JsonProperty("scheduled_message")
   private ScheduledMessage scheduledMessage;
 
-  /** On scheduled_message.sent, the ID of the message that was sent. */
+  @NotNull @JsonIgnore private Map<String, Object> additionalFields = new HashMap<>();
+
+  /**
+   * The ID of the message the event is about (on scheduled_message.sent, the message that was
+   * sent). The backend also sends it on message.*, reminder.* and poll.* events.
+   *
+   * <p>Backed by {@link #getAdditionalFields()} so {@code getAdditionalFields().get("message_id")}
+   * keeps working for existing events.
+   */
   @Nullable
-  @JsonProperty("message_id")
-  private String messageId;
+  @JsonIgnore
+  public String getMessageId() {
+    Object value = this.additionalFields.get("message_id");
+    return value == null ? null : value.toString();
+  }
 
-  @NotNull @JsonIgnore private Map<String, Object> additionalFields = new HashMap<>();
+  /**
+   * Sets the ID of the message the event is about; stored in {@link #getAdditionalFields()}.
+   *
+   * @param messageId the message ID, or null to remove it
+   */
+  @JsonIgnore
+  public void setMessageId(@Nullable String messageId) {
+    if (messageId == null) {
+      this.additionalFields.remove("message_id");
+    } else {
+      this.additionalFields.put("message_id", messageId);
+    }
+  }
 
   @JsonAnyGetter
   public Map<String, Object> getAdditionalFields() {

src/test/java/io/getstream/chat/java/ScheduledMessageTest.java

@@ -180,7 +180,25 @@ void whenEventIsScheduledMessageSent_thenFieldsAreTyped() throws Exception {
             Event.class);
 
     Assertions.assertEquals("m-1", event.getMessageId());
+    Assertions.assertEquals("m-1", event.getAdditionalFields().get("message_id"));
     Assertions.assertEquals("sm-1", event.getScheduledMessage().getId());
     Assertions.assertFalse(event.getAdditionalFields().containsKey("scheduled_message"));
   }
+
+  @DisplayName("message.new events keep message_id in additional fields")
+  @Test
+  void whenEventIsMessageNew_thenMessageIdStaysInAdditionalFields() throws Exception {
+    String json =
+        "{\"type\":\"message.new\",\"cid\":\"messaging:general\",\"message_id\":\"m-2\","
+            + "\"message\":{\"id\":\"m-2\",\"text\":\"hi\"}}";
+    Event event = MAPPER.readValue(json, Event.class);
+
+    Assertions.assertEquals("m-2", event.getAdditionalFields().get("message_id"));
+    Assertions.assertEquals("m-2", event.getMessageId());
+    Assertions.assertEquals("m-2", event.getMessage().getId());
+
+    String out = MAPPER.writeValueAsString(event);
+    Assertions.assertEquals(1, out.split("\"message_id\"", -1).length - 1, out);
+    Assertions.assertFalse(out.contains("messageId"), out);
+  }
 }