Admin Code Suggestions - Add default injections

What does this MR do and why?

Closes #480521 (closed)

This change adds default values to various injected values in the code_suggestion domain to remove console errors.

thought: We have guidelines and lint rules around our props that disallows the array syntax when defining them. I'm really surprised we don't have any docs around Provide/Inject. We likely should always avoid the array syntax for provide/inject as well.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Setup GitLab Duo on your GDK by following these docs: https://docs.gitlab.com/ee/development/ai_features/index.html#option-b-run-gdk-in-self-managed-mode-and-enable-ai-features-for-the-instance
2. Navigate to admin/code_suggestions
3. Ensure no injection warnings are in the console

Related to #480521 (closed)