Ensure Bun Ecosystem is Placed Under Javascript in Image Builds

[kbukum1]

What are you trying to accomplish?

This PR ensures that the bun ecosystem is correctly placed under the javascript directory in the ecosystem image generation process.

Previously, bun was included in the main loop alongside other standalone ecosystems. However, bun is part of javascript and should be structured accordingly.

Key Changes:

• Preserved the existing loop for other ecosystems (no modifications to the main structure).
• Added a separate RUN command to create the bun directory under javascript (javascript/lib/dependabot/javascript/bun).
• Ensured bun.rb is placed inside javascript/lib/dependabot/javascript/bun.rb instead of being treated as a separate top-level ecosystem.

What issues does this affect or fix?

This improves consistency in ecosystem organization and aligns bun with other package managers under javascript.

Anything you want to highlight for special attention from reviewers?

• This change does not affect functionality, but ensures proper file structure for bun.
• The new structure ensures better separation of concerns for package managers under javascript.

How will you know you've accomplished your goal?

• The build process correctly places bun under javascript instead of treating it as a standalone ecosystem.
• The structure follows the intended ecosystem hierarchy.
• No functionality changes, ensuring no impact on runtime behavior.

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.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request.
• I have ensured that the code is well-documented and easy to understand.

[kbukum1]

Duplicate: #11555