test: cover admin ui resource handler registration

[amirdeljouyi]

Hi,

I added two focused tests for the UI resource handler configuration.

Impact on coverage:

• Covers the servlet and reactive resource handler registration paths in AdminServerUiAutoConfiguration, including the reactive resource handlers and the servlet resource handlers.
• Line coverage for AdminServerUiAutoConfiguration increases by 14.81%.

Why:

They make sure the servlet and reactive configs can register the main UI assets and extension asset handlers with the configured locations.

I hope you find these tests useful. 😄 Please let me know if you have any questions or concerns.

Thanks,