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/Environments/EnvironmentDocumentResponseTests.cs

@@ -0,0 +1,63 @@
+using MicCheck.Api.Environments;
+using MicCheck.Api.Segments;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Environments;
+
+[TestFixture]
+public class SegmentDocumentResponseTests
+{
+    [Test]
+    public void WhenMappingASegment_ThenOnlyTopLevelRulesAreIncluded()
+    {
+        var segment = new Segment { Id = 1, Name = "Beta Users", ProjectId = 1, CreatedAt = DateTimeOffset.UtcNow };
+        var topLevelRule = new SegmentRule { Id = 1, SegmentId = 1, Type = SegmentRuleType.All };
+        var nestedRule = new SegmentRule { Id = 2, SegmentId = 1, ParentRuleId = 1, Type = SegmentRuleType.Any };
+        segment.Rules.Add(topLevelRule);
+        segment.Rules.Add(nestedRule);
+
+        var response = SegmentDocumentResponse.From(segment);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Name, Is.EqualTo("Beta Users"));
+        Assert.That(response.Rules, Has.Count.EqualTo(1));
+        Assert.That(response.Rules[0].Id, Is.EqualTo(1));
+    }
+}
+
+[TestFixture]
+public class SegmentRuleDocumentResponseTests
+{
+    [Test]
+    public void WhenMappingARule_ThenTypeIsUpperInvariantAndChildRulesAreIncluded()
+    {
+        var rule = new SegmentRule { Id = 1, SegmentId = 1, Type = SegmentRuleType.Any };
+        var condition = new SegmentCondition { Id = 1, RuleId = 1, Property = "plan", Operator = SegmentConditionOperator.Equal, Value = "pro" };
+        var childRule = new SegmentRule { Id = 2, SegmentId = 1, ParentRuleId = 1, Type = SegmentRuleType.All };
+        rule.Conditions.Add(condition);
+        rule.ChildRules.Add(childRule);
+
+        var response = SegmentRuleDocumentResponse.From(rule);
+
+        Assert.That(response.Type, Is.EqualTo("ANY"));
+        Assert.That(response.Conditions, Has.Count.EqualTo(1));
+        Assert.That(response.Rules, Has.Count.EqualTo(1));
+        Assert.That(response.Rules[0].Id, Is.EqualTo(2));
+    }
+}
+
+[TestFixture]
+public class SegmentConditionDocumentResponseTests
+{
+    [Test]
+    public void WhenMappingACondition_ThenPropertyOperatorAndValueAreCopied()
+    {
+        var condition = new SegmentCondition { Id = 1, RuleId = 1, Property = "plan", Operator = SegmentConditionOperator.Contains, Value = "pro" };
+
+        var response = SegmentConditionDocumentResponse.From(condition);
+
+        Assert.That(response.Property, Is.EqualTo("plan"));
+        Assert.That(response.Operator, Is.EqualTo("Contains"));
+        Assert.That(response.Value, Is.EqualTo("pro"));
+    }
+}