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/Scanner/StellaOps.Scanner.WebService/Contracts/ProofSpineContracts.cs

@@ -129,10 +129,22 @@ public sealed record DsseEnvelopeDto
 public sealed record DsseSignatureDto
 {
     [JsonPropertyName("keyid")]
+    [JsonPropertyOrder(0)]
     public string KeyId { get; init; } = string.Empty;
 
     [JsonPropertyName("sig")]
+    [JsonPropertyOrder(1)]
     public string Sig { get; init; } = string.Empty;
+
+    [JsonPropertyName("algorithm")]
+    [JsonPropertyOrder(2)]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public string? Algorithm { get; init; }
+
+    [JsonPropertyName("signature")]
+    [JsonPropertyOrder(3)]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public string? Signature { get; init; }
 }
 
 public sealed record ProofSpineVerificationDto