Add unit tests for Environments namespace (0% to 99.5% coverage)

Covers EnvironmentService (CRUD, clone, list, find-by-key), the
EnvironmentsController orchestration layer (webhooks, audit logs,
identities/traits, feature states, feature segments, identity
segments), EnvironmentDocumentService/Controller, response mappers,
and request validators.

tests/api/MicCheck.Api.Tests.Unit/Environments/EnvironmentResponseTests.cs

@@ -0,0 +1,30 @@
+using NUnit.Framework;
+using AppEnvironment = MicCheck.Api.Environments.Environment;
+
+namespace MicCheck.Api.Tests.Unit.Environments;
+
+[TestFixture]
+public class EnvironmentResponseTests
+{
+    [Test]
+    public void WhenMappingAnEnvironment_ThenAllFieldsAreCopied()
+    {
+        var createdAt = DateTimeOffset.UtcNow;
+        var environment = new AppEnvironment
+        {
+            Id = 1,
+            Name = "Production",
+            ApiKey = "env-key-abc",
+            ProjectId = 2,
+            CreatedAt = createdAt
+        };
+
+        var response = MicCheck.Api.Environments.EnvironmentResponse.From(environment);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Name, Is.EqualTo("Production"));
+        Assert.That(response.ApiKey, Is.EqualTo("env-key-abc"));
+        Assert.That(response.ProjectId, Is.EqualTo(2));
+        Assert.That(response.CreatedAt, Is.EqualTo(createdAt));
+    }
+}