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.Tests/FakeGatewayTransport.cs

@@ -91,6 +91,19 @@ internal sealed class FakeGatewayTransport(MxGatewayClientOptions options) : IMx
     /// </summary>
     public Queue<Exception> CloseSessionExceptions { get; } = new();
 
+    /// <summary>
+    /// Gets or sets a value indicating whether thrown <see cref="RpcException"/>s are mapped
+    /// to <see cref="MxGatewayException"/> the way the production gRPC transport does. Lets
+    /// retry tests exercise the wrapped-exception predicate branch that runs in production.
+    /// </summary>
+    public bool MapTransportExceptions { get; set; }
+
+    /// <summary>
+    /// Gets or sets an optional hook awaited inside CloseSessionAsync after the call is
+    /// recorded; lets tests pause a close mid-flight to observe concurrent dispose.
+    /// </summary>
+    public Func<Task>? CloseSessionHook { get; set; }
+
     /// <summary>
     /// Gets the queue of exceptions to throw from InvokeAsync.
     /// </summary>
@@ -108,7 +121,7 @@ internal sealed class FakeGatewayTransport(MxGatewayClientOptions options) : IMx
         OpenSessionCalls.Add((request, callOptions));
         if (OpenSessionExceptions.TryDequeue(out Exception? exception))
         {
-            throw exception;
+            throw Translate(exception, callOptions);
         }
 
         return Task.FromResult(OpenSessionReply);
@@ -119,17 +132,23 @@ internal sealed class FakeGatewayTransport(MxGatewayClientOptions options) : IMx
     /// </summary>
     /// <param name="request">The CloseSessionRequest to process.</param>
     /// <param name="callOptions">Call options specifying RPC behavior.</param>
-    public Task<CloseSessionReply> CloseSessionAsync(
+    public async Task<CloseSessionReply> CloseSessionAsync(
         CloseSessionRequest request,
         CallOptions callOptions)
     {
         CloseSessionCalls.Add((request, callOptions));
-        if (CloseSessionExceptions.TryDequeue(out Exception? exception))
+
+        if (CloseSessionHook is not null)
         {
-            throw exception;
+            await CloseSessionHook().ConfigureAwait(false);
         }
 
-        return Task.FromResult(CloseSessionReply);
+        if (CloseSessionExceptions.TryDequeue(out Exception? exception))
+        {
+            throw Translate(exception, callOptions);
+        }
+
+        return CloseSessionReply;
     }
 
     /// <summary>
@@ -144,7 +163,7 @@ internal sealed class FakeGatewayTransport(MxGatewayClientOptions options) : IMx
         InvokeCalls.Add((request, callOptions));
         if (InvokeExceptions.TryDequeue(out Exception? exception))
         {
-            throw exception;
+            throw Translate(exception, callOptions);
         }
 
         return Task.FromResult(_invokeReplies.Dequeue());
@@ -239,4 +258,18 @@ internal sealed class FakeGatewayTransport(MxGatewayClientOptions options) : IMx
     {
         _activeAlarmSnapshots.Add(snapshot);
     }
+
+    /// <summary>
+    /// Maps a queued exception the way the production gRPC transport does when
+    /// <see cref="MapTransportExceptions"/> is set; otherwise returns it unchanged.
+    /// </summary>
+    private Exception Translate(Exception exception, CallOptions callOptions)
+    {
+        if (MapTransportExceptions && exception is RpcException rpcException)
+        {
+            return RpcExceptionMapper.Map(rpcException, callOptions.CancellationToken);
+        }
+
+        return exception;
+    }
 }