Register Julia to register production check

[robaiken]

What are you trying to accomplish?

Fix failing Julia ecosystem tests by registering the required production check and label details for the "julia" package manager.

Anything you want to highlight for special attention from reviewers?

This follows the exact same pattern used by all other ecosystems (bundler, go_modules, etc.) by adding the registration calls at the end of the main module file

How will you know you've accomplished your goal?

The Julia ecosystem tests now pass successfully:

• The shared example "it registers the required classes" no longer raises "Unsupported package_manager julia" errors
• The Julia ecosystem is properly integrated with Dependabot's core infrastructure

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.