Ensure alert banners are announced on portfolio members table

[SamiyahKey]

Issue description

As a user navigating the Portfolio Members table with assistive technology,
I need error messages to be implemented properly so that they are announced when they appear,
so that I am aware of validation issues without having to manually search for them.

Acceptance criteria

• Implement role="alert" on alert elements (errors, success, etc) to ensure they are properly communicated.

• Avoid using display: none; on error elements, as hidden elements are not detected by screen readers.

• Ensure announcements work across different screen readers and browsers.

Additional context

No response

Links to other issues

No response

Implementation notes:

Copy or re-use implementation of alerts on new name server form (build the whole alert in JS). Per USWDS, using a display-none on an alert on page is counter-indicated.

We should be able to re-use name server alert JS by moving it or part of it into a helper file.