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/helper/actionCable.js

@@ -13,6 +13,7 @@ import {
 } from 'dashboard/composables/useWhatsappCallSession';
 import { VOICE_CALL_PROVIDERS } from 'dashboard/helper/inbox';
 import { VOICE_CALL_DIRECTION } from 'dashboard/components-next/message/constants';
+import { FEATURE_FLAGS } from 'dashboard/featureFlags';
 
 const { isImpersonating } = useImpersonation();
 const UNREAD_COUNTS_REFETCH_THROTTLE_MS = 5000;
@@ -171,10 +172,23 @@ class ActionCableConnector extends BaseActionCableConnector {
   };
 
   fetchConversationUnreadCounts = () => {
+    if (!this.isConversationUnreadCountsEnabled()) return;
+
     this.lastUnreadCountsFetchAt = Date.now();
     this.app.$store.dispatch('conversationUnreadCounts/get');
   };
 
+  isConversationUnreadCountsEnabled = () => {
+    const accountId = this.app.$store.getters.getCurrentAccountId;
+    const isFeatureEnabled =
+      this.app.$store.getters['accounts/isFeatureEnabledonAccount'];
+
+    return isFeatureEnabled?.(
+      accountId,
+      FEATURE_FLAGS.CONVERSATION_UNREAD_COUNTS
+    );
+  };
+
   onTypingOn = ({ conversation, user }) => {
     const conversationId = conversation.id;