Refactor SurfaceCacheValidator to simplify oldest entry calculation

Add global using for Xunit in test project

Enhance ImportValidatorTests with async validation and quarantine checks

Implement FileSystemQuarantineServiceTests for quarantine functionality

Add integration tests for ImportValidator to check monotonicity

Create BundleVersionTests to validate version parsing and comparison logic

Implement VersionMonotonicityCheckerTests for monotonicity checks and activation logic

src/Findings/StellaOps.Findings.Ledger/Services/IAuditService.cs

@@ -0,0 +1,17 @@
+using StellaOps.Findings.Ledger.Domain;
+
+namespace StellaOps.Findings.Ledger.Services;
+
+/// <summary>
+/// Service for audit timeline retrieval.
+/// </summary>
+public interface IAuditService
+{
+    /// <summary>
+    /// Gets the audit timeline for an alert.
+    /// </summary>
+    Task<AuditTimeline?> GetTimelineAsync(
+        string tenantId,
+        string alertId,
+        CancellationToken cancellationToken = default);
+}