chore: fix some async call issues in the .NET wrapper

spannerlib/wrappers/spannerlib-dotnet/spannerlib-dotnet-grpc-impl/GrpcBidiConnection.cs

@@ -58,8 +58,8 @@ protected override void CloseLibObject()
     private ConnectionStreamResponse ExecuteBidi(ConnectionStreamRequest request)
     {
         var connectionStream = Stream;
-        connectionStream.RequestStream.WriteAsync(request).GetAwaiter().GetResult();
-        if (!connectionStream.ResponseStream.MoveNext(CancellationToken.None).GetAwaiter().GetResult())
+        Task.Run(() => connectionStream.RequestStream.WriteAsync(request)).GetAwaiter().GetResult();
+        if (!Task.Run(() => connectionStream.ResponseStream.MoveNext(CancellationToken.None)).GetAwaiter().GetResult())
         {
             // This should never happen assuming that the gRPC server is well-behaved.
             throw new SpannerException(new Status { Code = (int)Code.Internal, Message = "No response received" });

spannerlib/wrappers/spannerlib-dotnet/spannerlib-dotnet-grpc-impl/GrpcLibSpanner.cs

@@ -416,11 +416,18 @@ public void BeginTransaction(Connection connection, TransactionOptions transacti
 
     public async Task BeginTransactionAsync(Connection connection, TransactionOptions transactionOptions, CancellationToken cancellationToken = default)
     {
-        await TranslateException(() => Client.BeginTransactionAsync(new BeginTransactionRequest
+        try
         {
-            Connection = ToProto(connection),
-            TransactionOptions = transactionOptions,
-        })).ConfigureAwait(false);
+            await Client.BeginTransactionAsync(new BeginTransactionRequest
+            {
+                Connection = ToProto(connection),
+                TransactionOptions = transactionOptions,
+            }, cancellationToken: cancellationToken).ConfigureAwait(false);
+        }
+        catch (RpcException exception)
+        {
+            throw SpannerException.ToSpannerException(exception);
+        }
     }
 
     public CommitResponse? Commit(Connection connection)

spannerlib/wrappers/spannerlib-dotnet/spannerlib-dotnet-grpc-impl/StreamingRows.cs

@@ -164,7 +164,7 @@ private bool TryNextCached(out ListValue? result, CancellationToken cancellation
         _stream ??= _spanner.ContinueStreaming(SpannerConnection, Id);
         try
         {
-            var hasNext = Task.Run(() => Stream.ResponseStream.MoveNext()).ResultWithUnwrappedExceptions();
+            var hasNext = Task.Run(() => Stream.ResponseStream.MoveNext()).GetAwaiter().GetResult();
             if (!hasNext)
             {
                 MarkDone();