Add spec for missing binaries diagnostic

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What does this merge request do and why?

Closes #Issue 2232

Test Category	Requirement
Basic Functionality
	#success? returns true when all binaries exist
	#success? returns false when binaries missing
	#detail returns nil when all binaries present
	#detail returns helpful instructions when missing
Individual Binary Tests
	Gitaly binary missing detection
	GitLab Shell binary missing detection
	Workhorse binary missing detection
Special Cases
	Workhorse glob pattern - file exists
	Workhorse glob pattern - no files found
	GraphQL schema always passes
Configuration Handling
	OpenBao enabled - missing binary fails
	OpenBao disabled - missing binary ignored
	OpenBao enabled - included in error message
Test Patterns
	File system mocking (File.exist?, File.executable?)
	Configuration mocking (stub_gdk_yaml)
	PackageConfig mocking
Helper Methods
	mock_all_binaries_exist
	mock_all_binaries_missing
	mock_specific_binary_paths
	Shared examples for DRY testing
Message Verification
	Complete message format with heredoc
	Specific binary inclusion in messages
	Setup command suggestions

How to set up and validate locally

1. Make sure GDK repository is cloned to your local system.
2. Checkout 2232-missing-binaries-spec from https://gitlab.com/ramananthiruchi09/gitlab-development-kit
3. Run bundle exec rspec spec/lib/gdk/diagnostic/missing_binaries_spec.rb -fd to run the rspecs for the file

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 MR references an issue describing the change.
• 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.