Fix queue auth: use user-assigned managed identity

The preview slot has both system-assigned and user-assigned identities.
DefaultAzureCredential can't disambiguate, so use ManagedIdentityCredential
with the explicit client ID (matching how AzureStorageService does it).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: JudahGabriel-MS

apps/pwabuilder/Services/AzureStorageAnalysisJobQueue.cs

@@ -27,8 +27,11 @@ public AzureStorageAnalysisJobQueue(
         this.healthMonitor = healthMonitor;
         this.logger = logger;
 
+        var managedIdentityAppId = appSettings.Value.AzureManagedIdentityApplicationId;
+        var credential = new ManagedIdentityCredential(clientId: managedIdentityAppId);
+
         var queueUri = new Uri($"{appSettings.Value.AzureStorageQueueUri.TrimEnd('/')}/{QueueName}");
-        this.queueClient = new QueueClient(queueUri, new DefaultAzureCredential());
+        this.queueClient = new QueueClient(queueUri, credential);
     }
 
     /// <summary>