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/MxGatewayAuthenticationException.cs

@@ -1,3 +1,4 @@
+using Grpc.Core;
 using MxGateway.Contracts.Proto;
 
 namespace MxGateway.Client;
@@ -13,6 +14,7 @@ public sealed class MxGatewayAuthenticationException : MxGatewayException
     /// <param name="hResult">The HResult code, if available.</param>
     /// <param name="statuses">The MXAccess statuses, if available.</param>
     /// <param name="innerException">The underlying exception, if any.</param>
+    /// <param name="statusCode">The gRPC status code reported by the failed call, if available.</param>
     public MxGatewayAuthenticationException(
         string message,
         string? sessionId = null,
@@ -20,7 +22,8 @@ public sealed class MxGatewayAuthenticationException : MxGatewayException
         ProtocolStatus? protocolStatus = null,
         int? hResult = null,
         IReadOnlyList<MxStatusProxy>? statuses = null,
-        Exception? innerException = null)
+        Exception? innerException = null,
+        StatusCode? statusCode = null)
         : base(
             message,
             sessionId,
@@ -28,7 +31,8 @@ public sealed class MxGatewayAuthenticationException : MxGatewayException
             protocolStatus,
             hResult,
             statuses ?? [],
-            innerException)
+            innerException,
+            statusCode)
     {
     }
 }