Yarp static files proxying does not support handling 404s

When using the Yarp resource to proxy over static files, the proxy doesn't handle falling back to the designated 404.html page when a route fails to resolve.

We might have to figure out how to configure a rule for it on the Aspire side or update the Yarp proxy itself to handle redirecting to `404.html` instead of returnin the status code when the `USE_STATIC_FILES` flag is enabled.

Start at the Yarp resource path used with USE_STATIC_FILES and trace how an unresolved route is handled. Determine whether Aspire configuration can select 404.html or whether the Yarp proxy must redirect; done means unresolved static-file routes reach the designated 404.html page.

Written by the indexing model from the issue text.