Refactor nunit tests off EF InMemory provider and web server

Extract IMicCheckDbContext so DB access can be mocked with Moq instead
of the InMemory provider, per project testing guidelines. Add shared
async DbSet test doubles (TestSupport/) to back the mocks. Rewrite
HTTP-pipeline tests (WebApplicationFactory) to exercise controllers
and auth handlers directly instead of running the web server. Drop
DbContext/seeder tests that only made sense against a real provider,
and remove stale duplicate test files left over from a namespace move.

tests/api/MicCheck.Api.Tests.Unit/TestSupport/TestAsyncEnumerable.cs

@@ -0,0 +1,29 @@
+using System.Collections;
+using System.Linq.Expressions;
+
+namespace MicCheck.Api.Tests.Unit.TestSupport;
+
+public class TestAsyncEnumerable<T> : EnumerableQuery<T>, IAsyncEnumerable<T>, IQueryable<T>
+{
+    public TestAsyncEnumerable(IEnumerable<T> enumerable) : base(enumerable) { }
+
+    public TestAsyncEnumerable(Expression expression) : base(expression) { }
+
+    public IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default) =>
+        new TestAsyncEnumerator<T>(this.AsEnumerable().GetEnumerator());
+
+    IQueryProvider IQueryable.Provider => new TestAsyncQueryProvider<T>(this);
+}
+
+public sealed class TestAsyncEnumerator<T>(IEnumerator<T> inner) : IAsyncEnumerator<T>
+{
+    public T Current => inner.Current;
+
+    public ValueTask<bool> MoveNextAsync() => ValueTask.FromResult(inner.MoveNext());
+
+    public ValueTask DisposeAsync()
+    {
+        inner.Dispose();
+        return ValueTask.CompletedTask;
+    }
+}