fix: Search rake task causing Rails boot error (#12416)

## Description

This PR sets up an `Enterprise::Railtie` to correctly register rake
tasks in the `enterprise` namespace.
Previously, rake tasks under `enterprise/lib/tasks` were being eagerly
loaded at Rails boot, causing `undefined method 'namespace'` errors.

With this change, rake tasks are now registered only in the rake
context, avoiding boot-time issues and ensuring they are discoverable
with `bin/rake -T`.

**Tasks added:**

* `search:all` → Reindex messages for all accounts
* `search:account[ID]` → Reindex messages for a specific account

Fixes: #12414


Co-authored-by: Sojan Jose <sojan@pepalo.com>

enterprise/lib/tasks/search.rake

@@ -1,22 +1,4 @@
-module Tasks::SearchTaskHelpers
-  def check_opensearch_config
-    if ENV['OPENSEARCH_URL'].blank?
-      puts 'Skipping reindex as OPENSEARCH_URL is not configured'
-      return false
-    end
-    true
-  end
-
-  def reindex_account(account)
-    Messages::ReindexService.new(account: account).perform
-    puts "Reindex task queued for account #{account.id}"
-  end
-end
-
 namespace :search do
-  desc 'Reindex messages using searchkick'
-  include Tasks::SearchTaskHelpers
-
   desc 'Reindex messages for all accounts'
   task all: :environment do
     next unless check_opensearch_config
@@ -47,3 +29,16 @@ namespace :search do
     reindex_account(account)
   end
 end
+
+def check_opensearch_config
+  if ENV['OPENSEARCH_URL'].blank?
+    puts 'Skipping reindex as OPENSEARCH_URL is not configured'
+    return false
+  end
+  true
+end
+
+def reindex_account(account)
+  Messages::ReindexService.new(account: account).perform
+  puts "Reindex task queued for account #{account.id}"
+end