.NET Purview Middleware: Improve Background Job Runner Injection (#3256)

* Clean up background job dependency injection

* Fix xml documentation grammar

dotnet/src/Microsoft.Agents.AI.Purview/BackgroundJobRunner.cs

@@ -12,7 +12,7 @@ namespace Microsoft.Agents.AI.Purview;
 /// <summary>
 /// Service that runs jobs in background threads.
 /// </summary>
-internal sealed class BackgroundJobRunner
+internal sealed class BackgroundJobRunner : IBackgroundJobRunner
 {
     private readonly IChannelHandler _channelHandler;
     private readonly IPurviewClient _purviewClient;
@@ -70,4 +70,12 @@ internal sealed class BackgroundJobRunner
                 break;
         }
     }
+
+    /// <summary>
+    /// Shutdown the job runners.
+    /// </summary>
+    public async Task ShutdownAsync()
+    {
+        await this._channelHandler.StopAndWaitForCompletionAsync().ConfigureAwait(false);
+    }
 }