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/MockDbSetFactory.cs

@@ -0,0 +1,56 @@
+using System.Linq.Expressions;
+using MicCheck.Api.Data;
+using Microsoft.EntityFrameworkCore;
+using Moq;
+
+namespace MicCheck.Api.Tests.Unit.TestSupport;
+
+public static class MockDbSetFactory
+{
+    public static Mock<DbSet<T>> Create<T>(List<T> data) where T : class
+    {
+        var queryable = data.AsQueryable();
+        var mockSet = new Mock<DbSet<T>>();
+
+        mockSet.As<IAsyncEnumerable<T>>()
+            .Setup(m => m.GetAsyncEnumerator(It.IsAny<CancellationToken>()))
+            .Returns(() => new TestAsyncEnumerator<T>(data.GetEnumerator()));
+
+        mockSet.As<IQueryable<T>>().Setup(m => m.Provider).Returns(() => new TestAsyncQueryProvider<T>(data.AsQueryable().Provider));
+        mockSet.As<IQueryable<T>>().Setup(m => m.Expression).Returns(() => data.AsQueryable().Expression);
+        mockSet.As<IQueryable<T>>().Setup(m => m.ElementType).Returns(queryable.ElementType);
+        mockSet.As<IQueryable<T>>().Setup(m => m.GetEnumerator()).Returns(() => data.GetEnumerator());
+
+        mockSet.Setup(m => m.Add(It.IsAny<T>())).Callback<T>(item => data.Add(item));
+        mockSet.Setup(m => m.AddRange(It.IsAny<IEnumerable<T>>())).Callback<IEnumerable<T>>(items => data.AddRange(items));
+        mockSet.Setup(m => m.Remove(It.IsAny<T>())).Callback<T>(item => data.Remove(item));
+        mockSet.Setup(m => m.RemoveRange(It.IsAny<IEnumerable<T>>())).Callback<IEnumerable<T>>(items =>
+        {
+            foreach (var item in items.ToList())
+                data.Remove(item);
+        });
+
+        return mockSet;
+    }
+
+    public static void SetupDbSet<T>(this Mock<IMicCheckDbContext> context, Expression<Func<IMicCheckDbContext, DbSet<T>>> selector, List<T> data)
+        where T : class =>
+        context.Setup(selector).Returns(Create(data).Object);
+
+    /// <summary>
+    /// Sets up a DbSet whose Add() simulates database-generated identity assignment (needed because
+    /// Moq does not run SaveChanges and entity Id properties are commonly init-only).
+    /// </summary>
+    public static void SetupDbSetWithGeneratedIds<T>(this Mock<IMicCheckDbContext> context, Expression<Func<IMicCheckDbContext, DbSet<T>>> selector, List<T> data)
+        where T : class
+    {
+        var idProperty = typeof(T).GetProperty("Id") ?? throw new InvalidOperationException($"{typeof(T).Name} has no Id property.");
+        var mockSet = Create(data);
+        mockSet.Setup(m => m.Add(It.IsAny<T>())).Callback<T>(item =>
+        {
+            idProperty.SetValue(item, data.Count + 1);
+            data.Add(item);
+        });
+        context.Setup(selector).Returns(mockSet.Object);
+    }
+}