Implement jwks endpoint and add test for it.

This also let to some improvements/cleanups of the other tests and fixtures.

IdentityShroud.Api/Apis/Filters/SlugValidationFilter.cs

@@ -0,0 +1,26 @@
+using IdentityShroud.Core.Model;
+using IdentityShroud.Core.Services;
+
+namespace IdentityShroud.Api;
+
+/// <summary>
+/// Note the filter depends on the slug path parameter to be the first string argument on the context.
+/// The endpoint handlers should place path arguments first and in order of the path to ensure this works
+/// consistently.
+/// </summary>
+/// <param name="realmService"></param>
+public class SlugValidationFilter(IRealmService realmService) : IEndpointFilter
+{
+    public async ValueTask<object?> InvokeAsync(EndpointFilterInvocationContext context, EndpointFilterDelegate next)
+    {
+        string slug = context.Arguments.OfType<string>().First();
+        Realm? realm = await realmService.FindBySlug(slug);
+        if (realm is null)
+        {
+            return Results.NotFound();
+        }
+        context.HttpContext.Items["RealmEntity"] = realm;
+
+        return await next(context);
+    }
+}