Address PR review: visibility key, null-label guard, test strictness

- Messages.kt: switch the pill's separator-visibility check to compare
  LazyListItemInfo.key against the separator item's id, instead of comparing
  the message-list index against the LazyList index. The two diverge when
  non-message items (footerContent, the load-more indicator) precede
  itemsIndexed in the column, which would mis-judge pill visibility.

- MessageListController.disableUnreadLabelButton: early-return when there is
  no active unread label. The previous code emitted false on
  showUnreadButtonState even on a no-op call (e.g. scrollToFirstUnreadMessage
  with a null unreadLabelState, or the XML SDK's HideUnreadLabel event with
  no active label), permanently latching the suppression for the controller's
  lifetime and hiding any future unread pill.

- MessageListControllerTests: strengthen the post-read assertion to fail when
  unreadLabelState becomes null. The previous safe-call chain silently passed
  when the chain returned null, masking the regression the test is meant to
  guard.

Author: andremion

stream-chat-android-compose/src/main/java/io/getstream/chat/android/compose/ui/messages/list/Messages.kt

@@ -386,13 +386,13 @@ internal fun BoxScope.DefaultMessagesHelperContent(
     )
 
     val unreadLabel = messagesState.unreadLabel
-    val unreadSeparatorIndex = remember(messages) {
-        messages.indexOfFirst { it is UnreadSeparatorItemState }
+    val unreadSeparatorId = remember(messages) {
+        messages.firstOrNull { it is UnreadSeparatorItemState }?.id
     }
-    val isUnreadSeparatorVisible by remember(unreadSeparatorIndex) {
+    val isUnreadSeparatorVisible by remember(unreadSeparatorId, lazyListState) {
         derivedStateOf {
-            unreadSeparatorIndex >= 0 &&
-                lazyListState.layoutInfo.visibleItemsInfo.any { it.index == unreadSeparatorIndex }
+            unreadSeparatorId != null &&
+                lazyListState.layoutInfo.visibleItemsInfo.any { it.key == unreadSeparatorId }
         }
     }
     val scrollToFirstUnreadVisible = isScrollToFirstUnreadVisible(

stream-chat-android-ui-common/src/main/kotlin/io/getstream/chat/android/ui/common/feature/messages/list/MessageListController.kt

@@ -636,8 +636,9 @@ public class MessageListController(
      * i.e. when the user leaves and re-enters the channel.
      */
     public fun disableUnreadLabelButton() {
+        val currentLabel = unreadLabelState.value ?: return
         showUnreadButtonState.tryEmit(false)
-        unreadLabelState.value = unreadLabelState.value?.copy(buttonVisibility = false)
+        unreadLabelState.value = currentLabel.copy(buttonVisibility = false)
     }
 
     /**

stream-chat-android-ui-common/src/test/kotlin/io/getstream/chat/android/ui/common/feature/messages/list/MessageListControllerTests.kt

@@ -1047,7 +1047,9 @@ internal class MessageListControllerTests {
 
         // The recomputed label must keep buttonVisibility false; otherwise the pill returns
         // immediately after the user dismisses or scrolls.
-        controller.unreadLabelState.value?.buttonVisibility?.`should be false`()
+        val unreadLabelAfterRead = controller.unreadLabelState.value
+        unreadLabelAfterRead.shouldNotBeNull()
+        unreadLabelAfterRead.buttonVisibility.`should be false`()
     }
 
     @Test