Add tests and implement StubBearer authentication for Signer endpoints

- Created SignerEndpointsTests to validate the SignDsse and VerifyReferrers endpoints.
- Implemented StubBearerAuthenticationDefaults and StubBearerAuthenticationHandler for token-based authentication.
- Developed ConcelierExporterClient for managing Trivy DB settings and export operations.
- Added TrivyDbSettingsPageComponent for UI interactions with Trivy DB settings, including form handling and export triggering.
- Implemented styles and HTML structure for Trivy DB settings page.
- Created NotifySmokeCheck tool for validating Redis event streams and Notify deliveries.

src/StellaOps.Cli.Tests/Commands/CommandHandlersTests.cs

@@ -951,6 +951,15 @@ public sealed class CommandHandlersTests
 
         public Task<RuntimePolicyEvaluationResult> EvaluateRuntimePolicyAsync(RuntimePolicyEvaluationRequest request, CancellationToken cancellationToken)
             => Task.FromResult(RuntimePolicyResult);
+
+        public Task<OfflineKitDownloadResult> DownloadOfflineKitAsync(string? bundleId, string destinationDirectory, bool overwrite, bool resume, CancellationToken cancellationToken)
+            => throw new NotSupportedException();
+
+        public Task<OfflineKitImportResult> ImportOfflineKitAsync(OfflineKitImportRequest request, CancellationToken cancellationToken)
+            => throw new NotSupportedException();
+
+        public Task<OfflineKitStatus> GetOfflineKitStatusAsync(CancellationToken cancellationToken)
+            => throw new NotSupportedException();
     }
 
     private sealed class StubExecutor : IScannerExecutor