Add unit tests for MicCheck.Api.Identities

Covers AdminIdentityService (list/create/find/upsert-trait/delete-trait/
delete/feature-state get-set-delete, all previously 0%), the
AdminIdentityResponse mapping, TraitInput's JSON/plain-type branch
logic, and IdentitiesController.GetByIdentifier.

Raises the namespace from 31% to 97.5%.

tests/api/MicCheck.Api.Tests.Unit/Identities/AdminIdentityResponseTests.cs

@@ -0,0 +1,34 @@
+using MicCheck.Api.Identities;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Identities;
+
+[TestFixture]
+public class AdminIdentityResponseTests
+{
+    [Test]
+    public void WhenMappingAnIdentityWithNoTraits_ThenTheResponseHasAnEmptyTraitsList()
+    {
+        var identity = new Identity { Id = 1, Identifier = "user-1", EnvironmentId = 5, CreatedAt = DateTimeOffset.UtcNow };
+
+        var response = AdminIdentityResponse.From(identity);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Identifier, Is.EqualTo("user-1"));
+        Assert.That(response.EnvironmentId, Is.EqualTo(5));
+        Assert.That(response.Traits, Is.Empty);
+    }
+
+    [Test]
+    public void WhenMappingAnIdentityWithTraits_ThenEachTraitIsMapped()
+    {
+        var identity = new Identity { Id = 1, Identifier = "user-1", EnvironmentId = 1, CreatedAt = DateTimeOffset.UtcNow };
+        identity.Traits.Add(new IdentityTrait { Key = "plan", Value = "premium" });
+
+        var response = AdminIdentityResponse.From(identity);
+
+        Assert.That(response.Traits, Has.Count.EqualTo(1));
+        Assert.That(response.Traits[0].Key, Is.EqualTo("plan"));
+        Assert.That(response.Traits[0].Value, Is.EqualTo("premium"));
+    }
+}