Resolve Client.Dotnet-001, -002, -003 code-review findings

Client.Dotnet-001: MapRpcException typed only Unauthenticated and
PermissionDenied; every other gRPC status collapsed to an untyped exception
with the status code discarded. Added a nullable StatusCode to
MxGatewayException, extracted the duplicated mappers into a shared
RpcExceptionMapper that records the code for every status, and documented it.

Client.Dotnet-002: the production retry branch (MxGatewayException wrapping
RpcException) was never exercised. FakeGatewayTransport gained a
MapTransportExceptions mode that runs thrown RpcExceptions through
RpcExceptionMapper exactly as the production transport does.

Client.Dotnet-003: MxGatewaySession.DisposeAsync disposed _closeLock while a
concurrent CloseAsync could be parked in WaitAsync. DisposeAsync now drains
in-flight CloseAsync callers before disposing the semaphore; the client's
_disposed flag is accessed via Interlocked.

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

clients/dotnet/MxGateway.Client/MxGatewayClient.cs

@@ -17,7 +17,7 @@ public sealed class MxGatewayClient : IAsyncDisposable
     private readonly GrpcChannel _channel;
     private readonly IMxGatewayClientTransport _transport;
     private readonly ResiliencePipeline _safeUnaryRetryPipeline;
-    private bool _disposed;
+    private int _disposed;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="MxGatewayClient"/> with given options and transport.
@@ -229,12 +229,11 @@ public sealed class MxGatewayClient : IAsyncDisposable
     /// </summary>
     public ValueTask DisposeAsync()
     {
-        if (_disposed)
+        if (Interlocked.Exchange(ref _disposed, 1) != 0)
         {
             return ValueTask.CompletedTask;
         }
 
-        _disposed = true;
         _channel?.Dispose();
         return ValueTask.CompletedTask;
     }
@@ -335,6 +334,6 @@ public sealed class MxGatewayClient : IAsyncDisposable
 
     private void ThrowIfDisposed()
     {
-        ObjectDisposedException.ThrowIf(_disposed, this);
+        ObjectDisposedException.ThrowIf(Volatile.Read(ref _disposed) != 0, this);
     }
 }