Fixed invalid states for custom stage fields in VSA settings form

What does this MR do and why?

Improved invalid states for custom stage fields in the Value Stream Analytics settings form to align with Pajamas Design guidelines.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• #415611 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
Screen Recording 2025-01-23 at 10.33.23 PM.mov	Screen Recording 2025-01-27 at 2.47.24 PM.mov

How to set up and validate locally

1. Please make sure that you're on the GitLab Ultimate plan.
2. Visit any group/project, then navigate to Analyze > Value Stream Analytics in the menu
3. Select New value stream...
4. On the settings page, begin filling out the form then select Add stage. Verify that the custom stage fields are in invalid states.

Related to #415611 (closed)