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/RpcExceptionMapperTests.cs

@@ -0,0 +1,76 @@
+using Grpc.Core;
+
+namespace MxGateway.Client.Tests;
+
+/// <summary>Tests for the shared gRPC-to-native exception mapping used by the transports.</summary>
+public sealed class RpcExceptionMapperTests
+{
+    /// <summary>Verifies that an unauthenticated status maps to the authentication exception.</summary>
+    [Fact]
+    public void Map_UnauthenticatedStatus_ProducesAuthenticationException()
+    {
+        RpcException rpc = new(new Status(StatusCode.Unauthenticated, "no key"));
+
+        Exception mapped = RpcExceptionMapper.Map(rpc, CancellationToken.None);
+
+        MxGatewayAuthenticationException authentication =
+            Assert.IsType<MxGatewayAuthenticationException>(mapped);
+        Assert.Equal(StatusCode.Unauthenticated, authentication.StatusCode);
+    }
+
+    /// <summary>Verifies that a permission-denied status maps to the authorization exception.</summary>
+    [Fact]
+    public void Map_PermissionDeniedStatus_ProducesAuthorizationException()
+    {
+        RpcException rpc = new(new Status(StatusCode.PermissionDenied, "missing scope"));
+
+        Exception mapped = RpcExceptionMapper.Map(rpc, CancellationToken.None);
+
+        MxGatewayAuthorizationException authorization =
+            Assert.IsType<MxGatewayAuthorizationException>(mapped);
+        Assert.Equal(StatusCode.PermissionDenied, authorization.StatusCode);
+    }
+
+    /// <summary>Verifies that a cancelled status maps to OperationCanceledException.</summary>
+    [Fact]
+    public void Map_CancelledStatus_ProducesOperationCanceledException()
+    {
+        RpcException rpc = new(new Status(StatusCode.Cancelled, "cancelled"));
+
+        Exception mapped = RpcExceptionMapper.Map(rpc, CancellationToken.None);
+
+        Assert.IsType<OperationCanceledException>(mapped);
+    }
+
+    /// <summary>
+    /// Verifies that non-auth statuses surface the originating gRPC status code on the
+    /// mapped exception so callers can distinguish transient from permanent failures
+    /// without reflecting into InnerException.
+    /// </summary>
+    [Theory]
+    [InlineData(StatusCode.NotFound)]
+    [InlineData(StatusCode.InvalidArgument)]
+    [InlineData(StatusCode.ResourceExhausted)]
+    [InlineData(StatusCode.FailedPrecondition)]
+    [InlineData(StatusCode.Unavailable)]
+    [InlineData(StatusCode.Internal)]
+    public void Map_NonAuthStatus_CarriesStatusCodeOnMxGatewayException(StatusCode statusCode)
+    {
+        RpcException rpc = new(new Status(statusCode, "boom"));
+
+        Exception mapped = RpcExceptionMapper.Map(rpc, CancellationToken.None);
+
+        MxGatewayException gatewayException = Assert.IsType<MxGatewayException>(mapped);
+        Assert.Equal(statusCode, gatewayException.StatusCode);
+        Assert.Same(rpc, gatewayException.InnerException);
+    }
+
+    /// <summary>Verifies that an MxGatewayException built without a gRPC status reports a null StatusCode.</summary>
+    [Fact]
+    public void StatusCode_IsNull_WhenNoGrpcStatusProvided()
+    {
+        MxGatewayException gatewayException = new("plain failure");
+
+        Assert.Null(gatewayException.StatusCode);
+    }
+}