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/Common/Security/Authentication/ApiKeyAuthenticationHandlerTests.cs

@@ -75,6 +75,14 @@ public class ApiKeyAuthenticationHandlerTests
         Assert.That(result.None, Is.True);
     }
 
+    [Test]
+    public async Task WhenApiKeyValueIsEmpty_ThenAuthenticationFails()
+    {
+        var result = await AuthenticateAsync("Api-Key ");
+
+        Assert.That(result.Succeeded, Is.False);
+    }
+
     [Test]
     public async Task WhenApiKeyIsInvalid_ThenAuthenticationFails()
     {

tests/api/MicCheck.Api.Tests.Unit/Common/Security/Authentication/EnvironmentKeyAuthenticationHandlerTests.cs

@@ -51,6 +51,14 @@ public class EnvironmentKeyAuthenticationHandlerTests
         Assert.That(result.None, Is.True);
     }
 
+    [Test]
+    public async Task WhenEnvironmentKeyHeaderIsEmpty_ThenAuthenticationFails()
+    {
+        var result = await AuthenticateAsync("");
+
+        Assert.That(result.Succeeded, Is.False);
+    }
+
     [Test]
     public async Task WhenEnvironmentKeyIsInvalid_ThenAuthenticationFails()
     {