Add an all seats used for non restricted access cases

What does this MR do and why?

Relates to Implement an all seats used notification for no... (#523557 - closed)

This change adds a new warning alert that appears when a GitLab group has used up all available seats in their subscription. The system now shows two different messages depending on the group's settings:

1. If Restricted access is enabled, it shows the existing "all seats used" alert that prevents adding new users
2. If Restricted access is disabled, it shows a new "no more seats" alert that warns about potential overage charges and suggests either enabling restricted access or purchasing more seats

The new alert includes helpful links to documentation about managing seats and turning on access restrictions. The change also updates the display logic so only group owners see these alerts, and includes comprehensive tests to ensure the alerts appear correctly based on different scenarios like user permissions, subscription status, and group settings.

This improvement helps group administrators better understand their seat usage situation and provides clear guidance on how to manage their subscription limits.

Note: The Alert is currently dismissible. A callout functionality will be added on a follow-up MR.

References

Screenshots or screen recordings

Before	After
Threshold banner (Open access)
	same
Threshold banner (Restricted access)
	same
All seats used banner (Open access)
N/A
All seats used banner (Restricted access)
	same

How to set up and validate locally

MR acceptance checklist

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