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/Webhooks/CreateWebhookRequestValidatorTests.cs

@@ -0,0 +1,46 @@
+using MicCheck.Api.Webhooks;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Webhooks;
+
+[TestFixture]
+public class CreateWebhookRequestValidatorTests
+{
+    private CreateWebhookRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new CreateWebhookRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new CreateWebhookRequest("https://example.com/hook", "secret", true));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenUrlIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateWebhookRequest("", null, true));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenUrlExceedsMaximumLength_ThenValidationFails()
+    {
+        var longUrl = "https://example.com/" + new string('a', 500);
+        var result = _validator.Validate(new CreateWebhookRequest(longUrl, null, true));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenUrlIsNotAnAbsoluteUri_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateWebhookRequest("not a valid url", null, true));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+}