Fix/non asset framework files

[NetherGranite]

This is a draft implementation of #66059.

[dotnet-policy-service]

Thanks for your PR, @@NetherGranite. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[NetherGranite]

@dotnet-policy-service agree

[javiercn]

Thanks for the contribution, and for the detailed write-up and history on #66059. We are going to hold off on this one, because the diagnosis it is built on does not match what happens today: blazor.web.js and blazor.server.js are shipped as static web assets, and app.UseStaticFiles() already serves them without MapStaticAssets() (this is what our own server-side Blazor E2E tests exercise, using UseStaticFiles() and MapBlazorHub() with no MapStaticAssets() call). The endpoints removed in #58721 were serving those files from resources embedded in the framework assemblies, which we intentionally stopped doing so the files can be fingerprinted and compressed like any other asset. The failures in #64381 come from the framework JS not being brought into the app as a static web asset in the first place (for example RCL-only hosts, Blazor Hybrid, or library output), so re-adding endpoints that read from the web root would still return 404 in exactly those cases, and would duplicate what UseStaticFiles() already does everywhere else. For hosts that need the framework JS but do not contain a .razor file, the supported way to bring the assets in is the RequiresAspNetWebAssets MSBuild property:

<PropertyGroup>
  <RequiresAspNetWebAssets>true</RequiresAspNetWebAssets>
</PropertyGroup>

Setting that forces the Blazor framework files to be included as static web assets even when the project has no components, after which UseStaticFiles() serves them as usual.