fix(Update-KHDB): dispose HttpResponseMessage on download

Invoke-DownloadWithRetry disposed the response stream and file handle
but never the HttpResponseMessage itself returned by GetAsync,
leaking one per shard download (the S3 code path already disposes
its response object). Added a finally block per attempt to dispose
it on both the success and retry/failure paths.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Update-KHDB.ps1

@@ -76,6 +76,7 @@ function Invoke-DownloadWithRetry {
     $retries = 5
     $delay = 2
     for ($attempt = 0; $attempt -lt $retries; $attempt++) {
+        $response = $null
         try {
             $response = $Client.GetAsync($Uri, [System.Net.Http.HttpCompletionOption]::ResponseHeadersRead).Result
             if (-not $response.IsSuccessStatusCode) {
@@ -115,6 +116,8 @@ function Invoke-DownloadWithRetry {
             } else {
                 throw
             }
+        } finally {
+            if ($response) { $response.Dispose() }
         }
     }
 }