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/CreateEnvironmentRequestValidatorTests.cs

@@ -0,0 +1,95 @@
+using MicCheck.Api.Environments;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Environments;
+
+[TestFixture]
+public class CreateEnvironmentRequestValidatorTests
+{
+    private CreateEnvironmentRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new CreateEnvironmentRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new CreateEnvironmentRequest("Staging", 1));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenNameIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateEnvironmentRequest("", 1));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameExceedsMaximumLength_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateEnvironmentRequest(new string('a', 201), 1));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenProjectIdIsZeroOrLess_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateEnvironmentRequest("Staging", 0));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+}
+
+[TestFixture]
+public class UpdateEnvironmentRequestValidatorTests
+{
+    private UpdateEnvironmentRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new UpdateEnvironmentRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new UpdateEnvironmentRequest("Renamed"));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenNameIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new UpdateEnvironmentRequest(""));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+}
+
+[TestFixture]
+public class CloneEnvironmentRequestValidatorTests
+{
+    private CloneEnvironmentRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new CloneEnvironmentRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new CloneEnvironmentRequest("Staging Copy"));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenNameIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CloneEnvironmentRequest(""));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+}