Add unit tests for Environments, Audit, and Webhooks namespaces (#6)

## Summary
- Add missing unit test coverage for the Environments, Audit, and Webhooks namespaces (raises them from ~0-62% to 91-100%)
- Exclude WebhookBackgroundService/WebhookRetryBackgroundService from coverage (require live DI/DB, disallowed by CLAUDE.md's no-InMemory/WebApplicationFactory rule)

## Test plan
- [x] `dotnet test` full suite passes (646/646)
- [x] Coverage report confirms Environments ~99.5%, Audit 100%, Webhooks ~91%

Reviewed-on: #6

tests/api/MicCheck.Api.Tests.Unit/Features/CreateFeatureRequestValidatorTests.cs

@@ -0,0 +1,61 @@
+using MicCheck.Api.Features;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class CreateFeatureRequestValidatorTests
+{
+    private CreateFeatureRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new CreateFeatureRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("dark_mode", FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenNameIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("", FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameExceedsMaximumLength_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest(new string('a', 151), FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameContainsInvalidCharacters_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("dark mode!", FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenInitialValueExceedsMaximumLength_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("flag", FeatureType.Standard, new string('a', 20_001), null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenInitialValueIsNull_ThenTheLengthRuleIsSkipped()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("flag", FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+}