🧹 Refactor: Extract shared error message in DeferredMultiNodeBackend

[edithatogo]

🎯 What: Extracted duplicate exception raising logic into a single private helper method _fail() within DeferredMultiNodeBackend. Both predict and design_matrix methods now call this helper.

💡 Why: Reduces duplicate code, making the exception message and failure logic a single source of truth for the deferred backend. This improves maintainability since future updates to the deferred messaging or logic only require changing one location.

✅ Verification:

• Activated UV virtual environment.
• Formatted and linted code with ruff check and ruff format (no errors).
• Ran tests with pytest verifying that the overall cluster features continue to function and the specific multi-node deferred backend raises the correct exception.

✨ Result: Cleaned up identical implementations of the NotImplementedError across both predict and design_matrix methods in pymars/cluster.py:143.

---

PR created automatically by Jules for task 16792646342385854045 started by @edithatogo

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[cursor]

Bugbot is not enabled for your account, so this pull request was not reviewed.

Enable Bugbot in the Cursor dashboard to get automatic reviews on future PRs.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[github-actions]

🎉 Welcome @edithatogo! Thank you for your first pull request to mars! We're excited to have you as a contributor. Our team will review your PR soon. In the meantime, please ensure all CI checks pass.