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/Segments/CreateSegmentRequestValidatorTests.cs

@@ -0,0 +1,120 @@
+using MicCheck.Api.Segments;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Segments;
+
+[TestFixture]
+public class CreateSegmentRequestValidatorTests
+{
+    private CreateSegmentRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new CreateSegmentRequestValidator();
+
+    private static CreateSegmentRequest ValidRequest() => new(
+        "Premium Users",
+        [new CreateSegmentRuleRequest("All", [new CreateSegmentConditionRequest("plan", "Equal", "premium")])]);
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(ValidRequest());
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenTheNameIsEmpty_ThenValidationFails()
+    {
+        var request = ValidRequest() with { Name = "" };
+
+        var result = _validator.Validate(request);
+
+        Assert.That(result.IsValid, Is.False);
+        Assert.That(result.Errors.Any(e => e.PropertyName == "Name"), Is.True);
+    }
+
+    [Test]
+    public void WhenTheNameExceedsMaximumLength_ThenValidationFails()
+    {
+        var request = ValidRequest() with { Name = new string('a', 201) };
+
+        var result = _validator.Validate(request);
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenRulesIsNull_ThenValidationFails()
+    {
+        var request = ValidRequest() with { Rules = null! };
+
+        var result = _validator.Validate(request);
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenARuleTypeIsInvalid_ThenValidationFails()
+    {
+        var request = new CreateSegmentRequest(
+            "Test",
+            [new CreateSegmentRuleRequest("NotAType", [new CreateSegmentConditionRequest("plan", "Equal", "premium")])]);
+
+        var result = _validator.Validate(request);
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [TestCase("All")]
+    [TestCase("Any")]
+    [TestCase("None")]
+    [TestCase("all")]
+    public void WhenARuleTypeIsAValidCaseInsensitiveName_ThenValidationSucceeds(string type)
+    {
+        var request = new CreateSegmentRequest(
+            "Test",
+            [new CreateSegmentRuleRequest(type, [new CreateSegmentConditionRequest("plan", "Equal", "premium")])]);
+
+        var result = _validator.Validate(request);
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenAConditionPropertyIsEmpty_ThenValidationFails()
+    {
+        var request = new CreateSegmentRequest(
+            "Test",
+            [new CreateSegmentRuleRequest("All", [new CreateSegmentConditionRequest("", "Equal", "premium")])]);
+
+        var result = _validator.Validate(request);
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenAConditionOperatorIsInvalid_ThenValidationFails()
+    {
+        var request = new CreateSegmentRequest(
+            "Test",
+            [new CreateSegmentRuleRequest("All", [new CreateSegmentConditionRequest("plan", "NotAnOperator", "premium")])]);
+
+        var result = _validator.Validate(request);
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenANestedChildRuleHasAnInvalidOperator_ThenValidationStillSucceedsBecauseChildRulesAreNotValidatedRecursively()
+    {
+        var childRule = new CreateSegmentRuleRequest("All", [new CreateSegmentConditionRequest("plan", "NotAnOperator", "premium")]);
+        var request = new CreateSegmentRequest(
+            "Test",
+            [new CreateSegmentRuleRequest("All", [new CreateSegmentConditionRequest("plan", "Equal", "premium")], [childRule])]);
+
+        var result = _validator.Validate(request);
+
+        Assert.That(result.IsValid, Is.True);
+    }
+}