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/Organizations/CreateOrganizationRequestValidatorTests.cs

@@ -0,0 +1,103 @@
+using MicCheck.Api.Organizations;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Organizations;
+
+[TestFixture]
+public class CreateOrganizationRequestValidatorTests
+{
+    private CreateOrganizationRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new CreateOrganizationRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new CreateOrganizationRequest("My Org"));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenNameIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateOrganizationRequest(""));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameExceedsMaximumLength_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateOrganizationRequest(new string('a', 201)));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+}
+
+[TestFixture]
+public class UpdateOrganizationRequestValidatorTests
+{
+    private UpdateOrganizationRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new UpdateOrganizationRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new UpdateOrganizationRequest("Renamed"));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenNameIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new UpdateOrganizationRequest(""));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+}
+
+[TestFixture]
+public class InviteUserRequestValidatorTests
+{
+    private InviteUserRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new InviteUserRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new InviteUserRequest(1, "Admin"));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenUserIdIsZeroOrLess_ThenValidationFails()
+    {
+        var result = _validator.Validate(new InviteUserRequest(0, "User"));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenRoleIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new InviteUserRequest(1, ""));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenRoleIsNotAValidOrganizationRole_ThenValidationFails()
+    {
+        var result = _validator.Validate(new InviteUserRequest(1, "SuperAdmin"));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+}