FIX: Rebake chat messages when GitHub PR status changes (#36561)

Fix GitHub PR onebox updates not working for chat messages.

When a GitHub webhook triggered the `RebakeGithubPrPosts` job, chat
messages containing PR links were not being rebaked. The issue was
twofold:

1. `defined?(Chat::MessageLink)` doesn't trigger Rails autoloading - it
only checks if a constant is already loaded, so it would return `nil`
before the class was ever accessed
2. Inside `module Jobs`, Ruby's constant resolution would look for
`Jobs::Chat::MessageLink` before the global `::Chat::MessageLink`

- Replace `defined?(Chat)` check with `SiteSetting.chat_enabled` which
is the idiomatic Discourse pattern for checking plugin availability
- Remove the redundant `defined?(Chat::MessageLink)` guard since the
site setting check already handles this
- Add `::` prefix to `Chat::MessageLink` and `Chat::Message` references
to ensure they resolve to the global namespace

Internal ref - t/169442

plugins/discourse-github/app/jobs/regular/rebake_github_pr_posts.rb

@@ -9,7 +9,7 @@ module Jobs
       return if pr_url.blank?
 
       rebake_posts(pr_url)
-      rebake_chat_messages(pr_url) if defined?(Chat)
+      rebake_chat_messages(pr_url) if SiteSetting.chat_enabled
     end
 
     private
@@ -30,15 +30,13 @@ module Jobs
     end
 
     def rebake_chat_messages(pr_url)
-      return unless defined?(Chat::MessageLink)
-
       message_ids =
-        Chat::MessageLink
+        ::Chat::MessageLink
           .where(url: pr_url)
-          .or(Chat::MessageLink.where("url LIKE ?", "#{pr_url}%"))
+          .or(::Chat::MessageLink.where("url LIKE ?", "#{pr_url}%"))
           .select(:chat_message_id)
 
-      Chat::Message
+      ::Chat::Message
         .where(id: message_ids)
         .find_each do |message|
           next unless has_github_pr_onebox?(message.cooked, pr_url)