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.

spec/controllers/api/v1/accounts/conversations_controller_spec.rb

@@ -126,32 +126,47 @@ RSpec.describe 'Conversations API', type: :request do
         Conversations::UnreadCounts::Store.clear_account!(account.id)
       end
 
-      it 'returns unread conversation counts scoped to the signed-in user' do
-        create_unread_conversation(account: account, inbox: visible_inbox, labels: [label.title])
-        create_unread_conversation(account: account, inbox: hidden_inbox, labels: [label.title])
+      context 'when conversation unread counts feature is enabled' do
+        before do
+          account.enable_features!(:conversation_unread_counts)
+        end
 
-        get "/api/v1/accounts/#{account.id}/conversations/unread_counts",
-            headers: agent.create_new_auth_token,
-            as: :json
+        it 'returns unread conversation counts scoped to the signed-in user' do
+          create_unread_conversation(account: account, inbox: visible_inbox, labels: [label.title])
+          create_unread_conversation(account: account, inbox: hidden_inbox, labels: [label.title])
 
-        expect(response).to have_http_status(:success)
-        expect(response.parsed_body['payload']).to eq(
-          'inboxes' => { visible_inbox.id.to_s => 1 },
-          'labels' => { label.id.to_s => 1 },
-          'teams' => {}
-        )
+          get "/api/v1/accounts/#{account.id}/conversations/unread_counts",
+              headers: agent.create_new_auth_token,
+              as: :json
+
+          expect(response).to have_http_status(:success)
+          expect(response.parsed_body['payload']).to eq(
+            'inboxes' => { visible_inbox.id.to_s => 1 },
+            'labels' => { label.id.to_s => 1 },
+            'teams' => {}
+          )
+        end
+
+        it 'returns unread team conversation counts scoped to the signed-in user' do
+          create_unread_conversation(account: account, inbox: visible_inbox, team: team)
+          create_unread_conversation(account: account, inbox: hidden_inbox, team: team)
+
+          get "/api/v1/accounts/#{account.id}/conversations/unread_counts",
+              headers: agent.create_new_auth_token,
+              as: :json
+
+          expect(response).to have_http_status(:success)
+          expect(response.parsed_body['payload']['teams']).to eq(team.id.to_s => 1)
+        end
       end
 
-      it 'returns unread team conversation counts scoped to the signed-in user' do
-        create_unread_conversation(account: account, inbox: visible_inbox, team: team)
-        create_unread_conversation(account: account, inbox: hidden_inbox, team: team)
-
+      it 'returns forbidden when conversation unread counts feature is disabled' do
         get "/api/v1/accounts/#{account.id}/conversations/unread_counts",
             headers: agent.create_new_auth_token,
             as: :json
 
-        expect(response).to have_http_status(:success)
-        expect(response.parsed_body['payload']['teams']).to eq(team.id.to_s => 1)
+        expect(response).to have_http_status(:forbidden)
+        expect(response.parsed_body['error']).to eq('Conversation unread counts feature not enabled for this account')
       end
     end
   end
@@ -833,6 +848,7 @@ RSpec.describe 'Conversations API', type: :request do
       end
 
       it 'refreshes unread count cache when conversation is marked read' do
+        account.enable_features!(:conversation_unread_counts)
         conversation.update!(agent_last_seen_at: 1.hour.ago)
         create(:message, account: account, inbox: conversation.inbox, conversation: conversation, message_type: :incoming, created_at: 5.minutes.ago)
         Conversations::UnreadCounts::Builder.new(account).build_base!
@@ -920,6 +936,7 @@ RSpec.describe 'Conversations API', type: :request do
       end
 
       it 'refreshes unread count cache when conversation is marked unread' do
+        account.enable_features!(:conversation_unread_counts)
         conversation.update!(agent_last_seen_at: 1.minute.from_now, assignee_last_seen_at: 1.minute.from_now)
         Conversations::UnreadCounts::Builder.new(account).build_base!