fix(export): mirror disposal fixes from #331

Port the same two BackupOperationBase / BackupClient fixes that landed in
#331 to their export counterparts (which were copy-pasted from the backup
pattern):

- ExportOperationBase.DisposeInternal: drop the self-Wait. The method is
  called from RefreshStatusInternal which itself runs inside
  _backgroundRefreshTask, so Wait()ing on that task blocked it on its own
  completion.
- ExportClient.CreateSync: wrap the operation in `await using` so it gets
  disposed on every exit path, not just success (where auto-dispose runs).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mpartipilo

src/Weaviate.Client/ExportClient.cs

@@ -70,7 +70,7 @@ public async Task<Export> CreateSync(
     )
     {
         await _client.EnsureVersion<ExportClient>();
-        var operation = await Create(request, cancellationToken);
+        await using var operation = await Create(request, cancellationToken);
         return await operation.WaitForCompletion(timeout, cancellationToken);
     }
 

src/Weaviate.Client/Models/ExportOperationBase.cs

@@ -163,18 +163,17 @@ protected virtual void Dispose(bool disposing)
         _disposed = true;
     }
 
+    /// <summary>
+    /// Internal disposal called from the background refresh task itself when a terminal status
+    /// is observed. Must NOT Wait() on _backgroundRefreshTask — that would block the very task
+    /// executing this code on its own completion (bounded only by the Wait timeout). The task
+    /// is already exiting because _cts has been canceled and _isCompleted is set, so just
+    /// release the CTS.
+    /// </summary>
     private void DisposeInternal()
     {
         if (_disposed)
             return;
-        try
-        {
-            _backgroundRefreshTask.Wait(ExportClient.Config.PollInterval);
-        }
-        catch (Exception)
-        {
-            // Best-effort disposal
-        }
         _cts.Dispose();
         _disposed = true;
     }