New MissingBinaries diagnostic

What does this merge request do and why?

Add a new MissingBinaries diagnostic to detect binaries that are missing. This helps catch issues where binaries weren't downloaded, for example, if gdk install didn't complete properly.

Closes #2320 (closed)

How to set up and validate locally

1. rm -rf gitlab/workhorse/gitlab-*

2. gdk doctor

   You should see this output:

   Missing Binaries
   ================================================================================
   The following binaries are missing from their expected paths:
     workhorse
   
   Please ensure you download them by running:
     make gitlab-workhorse-setup

After testing

1. Apply the following change:

   diff --git a/lib/gdk/package_helper.rb b/lib/gdk/package_helper.rb
   index 9fc6b0a9..5d18f1ab 100755
   --- a/lib/gdk/package_helper.rb
   +++ b/lib/gdk/package_helper.rb
   @@ -78,7 +78,7 @@ module GDK
           return
         end
   
   -      return GDK::Output.success("No changes detected in #{project_path}, skipping package download and extraction.") if current_commit_sha == stored_commit_sha
   +      # return GDK::Output.success("No changes detected in #{project_path}, skipping package download and extraction.") if current_commit_sha == stored_commit_sha
   
         uri = URI.parse("#{API_V4_URL}/projects/#{project_id}/packages/generic/#{package_name}/#{package_version}/#{File.basename(package_path)}")

2. make gitlab-workhorse-setup

This will restore gitlab-workhorse and get it running.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.