revert: restore conversation unread count feature flag (#14623)

This reverts #14610 so conversation unread counts are again controlled
by the `conversation_unread_counts` feature flag across the API,
ActionCable broadcasts, notifier/listener paths, and dashboard sidebar
fetching.

## Closes
- None

## What changed
- Restores feature-flag checks for conversation unread count reads and
broadcasts.
- Restores the dashboard feature flag constant and sidebar/store
behavior for disabled unread counts.
- Restores the specs that cover disabled-feature behavior.

## How to test
- In an account with `conversation_unread_counts` enabled, verify
sidebar unread counts are fetched and updated in real time.
- Disable `conversation_unread_counts` for the account and verify unread
count requests/broadcasts are skipped.

app/javascript/dashboard/components-next/sidebar/Sidebar.vue

@@ -61,9 +61,21 @@ const hasAdvancedAssignment = computed(() => {
   );
 });
 
-const fetchConversationUnreadCounts = currentAccountId => {
+const hasConversationUnreadCounts = computed(() => {
+  return isFeatureEnabledonAccount.value(
+    accountId.value,
+    FEATURE_FLAGS.CONVERSATION_UNREAD_COUNTS
+  );
+});
+
+const fetchConversationUnreadCounts = ([currentAccountId, isEnabled]) => {
   if (!currentAccountId) return;
 
+  if (!isEnabled) {
+    store.dispatch('conversationUnreadCounts/clear');
+    return;
+  }
+
   store.dispatch('conversationUnreadCounts/get');
 };
 
@@ -188,7 +200,7 @@ onMounted(() => {
   store.dispatch('customViews/get', 'contact');
 });
 
-watch(accountId, fetchConversationUnreadCounts, {
+watch([accountId, hasConversationUnreadCounts], fetchConversationUnreadCounts, {
   immediate: true,
 });