Fix admin redirects with conventional site routing

[MortenFriisSiteImprove]

Saving plugin settings can redirect to /SiteimproveAdmin/Index when the host registers a conventional {controller}/{action}/{id?} route, leading to a 404 outside the protected module.

Resolve the admin URL through IModuleResourceResolver for settings save and both prepublish configuration outcomes. Preserve prepublishError=true when enablement fails.

Related to #10. Keep the issue open until the containing package is published and its version is confirmed.

Validation:

• All 64 backend cases pass on each of .NET 6 and .NET 8.
• Eight HTTP regression cases cover save and prepublish enabled, disabled, and failure outcomes with and without a conventional route, using a custom module prefix. Five cases failed on each target before the fix and all pass afterward.
• Existing settings-persistence and authorization assertions pass.
• Tests use ASP.NET TestServer and a mocked module resolver; a live Optimizely CMS smoke test remains before release.