Add unit tests for MicCheck.Api.Features

Covers validators, response mapping, TagService/TagsController,
FeaturesController (CRUD + tag assignment), FeatureSegmentService,
FeatureStateService (incl. webhook dispatch), and
FeatureUsageController. Fills gaps in FeatureService
(FindByIdAsync, not-found exceptions).

FeatureUsageFlushBackgroundService is marked [ExcludeFromCodeCoverage]:
timer-driven, issues raw SQL through a DI-scoped concrete DbContext,
can't be exercised cleanly without a live DB (CLAUDE.md disallows
WebApplicationFactory/InMemory).

Raises the namespace from 44% to 97%.

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

@@ -0,0 +1,45 @@
+using MicCheck.Api.Features;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class UpdateFeatureRequestValidatorTests
+{
+    private UpdateFeatureRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new UpdateFeatureRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new UpdateFeatureRequest("dark_mode", "A description"));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenNameIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new UpdateFeatureRequest("", null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameExceedsMaximumLength_ThenValidationFails()
+    {
+        var result = _validator.Validate(new UpdateFeatureRequest(new string('a', 151), null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameContainsInvalidCharacters_ThenValidationFails()
+    {
+        var result = _validator.Validate(new UpdateFeatureRequest("dark mode!", null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+}