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/MxGatewayClientSessionTests.cs

@@ -231,6 +231,52 @@ public sealed class MxGatewayClientSessionTests
         Assert.Equal("session-fixture", call.Request.SessionId);
     }
 
+    /// <summary>
+    /// Verifies that disposing a session while other callers are concurrently inside
+    /// <see cref="MxGatewaySession.CloseAsync"/> — one holding the close lock and one
+    /// parked on it — never throws <see cref="ObjectDisposedException"/> into those
+    /// callers. The close lock must outlive every pending close.
+    /// </summary>
+    [Fact]
+    public async Task DisposeAsync_DoesNotRaceConcurrentCloseAsync()
+    {
+        for (int iteration = 0; iteration < 100; iteration++)
+        {
+            FakeGatewayTransport transport = CreateTransport();
+            using SemaphoreSlim firstCloseEntered = new(0, 1);
+            using SemaphoreSlim releaseFirstClose = new(0, 1);
+
+            // The first CloseAsync to reach the transport parks here while holding the
+            // session's close lock; later callers queue on the lock behind it.
+            transport.CloseSessionHook = async () =>
+            {
+                firstCloseEntered.Release();
+                await releaseFirstClose.WaitAsync().ConfigureAwait(false);
+                transport.CloseSessionHook = null;
+            };
+
+            await using MxGatewayClient client = CreateClient(transport);
+            MxGatewaySession session = await client.OpenSessionAsync();
+
+            // Holder enters CloseAsync, acquires the lock, and parks in the hook.
+            Task holder = Task.Run(() => session.CloseAsync());
+            await firstCloseEntered.WaitAsync();
+
+            // Waiter is parked on the close lock behind the holder.
+            Task waiter = Task.Run(() => session.CloseAsync());
+
+            // DisposeAsync runs concurrently; it must wait out both callers before
+            // disposing the close lock rather than tearing it down underneath them.
+            Task dispose = session.DisposeAsync().AsTask();
+
+            releaseFirstClose.Release();
+
+            await holder;
+            await waiter;
+            await dispose;
+        }
+    }
+
     /// <summary>Verifies that invoke retries safe diagnostic commands on transient RPC failure.</summary>
     [Fact]
     public async Task InvokeAsync_RetriesSafeDiagnosticCommandOnTransientGrpcFailure()
@@ -255,6 +301,35 @@ public sealed class MxGatewayClientSessionTests
         Assert.Equal(2, transport.InvokeCalls.Count);
     }
 
+    /// <summary>
+    /// Verifies that the retry pipeline still retries when the transport maps the raw
+    /// <see cref="RpcException"/> to an <see cref="MxGatewayException"/> before it reaches
+    /// the retry predicate — the wrapped-exception shape that production always produces.
+    /// </summary>
+    [Fact]
+    public async Task InvokeAsync_RetriesSafeDiagnosticCommand_WhenTransportMapsRpcException()
+    {
+        FakeGatewayTransport transport = CreateTransport();
+        transport.MapTransportExceptions = true;
+        transport.InvokeExceptions.Enqueue(CreateTransientRpcException());
+        transport.AddInvokeReply(new MxCommandReply
+        {
+            SessionId = "session-fixture",
+            Kind = MxCommandKind.Ping,
+            ProtocolStatus = new ProtocolStatus { Code = ProtocolStatusCode.Ok },
+        });
+        await using MxGatewayClient client = CreateClient(transport);
+        MxGatewaySession session = await client.OpenSessionAsync();
+
+        await session.InvokeAsync(new MxCommandRequest
+        {
+            SessionId = session.SessionId,
+            Command = new MxCommand { Kind = MxCommandKind.Ping, Ping = new PingCommand() },
+        });
+
+        Assert.Equal(2, transport.InvokeCalls.Count);
+    }
+
     /// <summary>Verifies that open session does not retry on transient RPC failure.</summary>
     [Fact]
     public async Task OpenSessionAsync_DoesNotRetryTransientGrpcFailure()