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.

src/api/MicCheck.Api/Features/FeatureSegmentService.cs

@@ -4,7 +4,7 @@ using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Features;
 
-public class FeatureSegmentService(MicCheckDbContext db)
+public class FeatureSegmentService(IMicCheckDbContext db)
 {
     public async Task<IReadOnlyList<FeatureSegmentResponse>> ListByFeatureAsync(
         int featureId, int environmentId, CancellationToken ct = default)