feat: Add VEX Status Chip component and integration tests for reachability drift detection

- Introduced `VexStatusChipComponent` to display VEX status with color coding and tooltips.
- Implemented integration tests for reachability drift detection, covering various scenarios including drift detection, determinism, and error handling.
- Enhanced `ScannerToSignalsReachabilityTests` with a null implementation of `ICallGraphSyncService` for better test isolation.
- Updated project references to include the new Reachability Drift library.

src/Scanner/StellaOps.Scanner.WebService/Services/PrAnnotationService.cs

@@ -518,18 +518,3 @@ public sealed class PrAnnotationService : IPrAnnotationService
         return purl[..47] + "...";
     }
 }
-
-/// <summary>
-/// Reachability state for a vulnerability (used by annotation service).
-/// </summary>
-public sealed record ReachabilityState
-{
-    public required string CveId { get; init; }
-    public required string Purl { get; init; }
-    public required bool IsReachable { get; init; }
-    public required string ConfidenceTier { get; init; }
-    public string? WitnessId { get; init; }
-    public string? Entrypoint { get; init; }
-    public string? FilePath { get; init; }
-    public int? LineNumber { get; init; }
-}