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/README.md

@@ -112,6 +112,17 @@ can keep the full `MxCommandReply`, HRESULT, and status array when MXAccess
 itself rejects a command. `MxAccessException.Reply` contains the raw generated
 reply.
 
+When a gRPC call itself fails, the transport maps the underlying
+`RpcException` to a native exception: `Unauthenticated` becomes
+`MxGatewayAuthenticationException`, `PermissionDenied` becomes
+`MxGatewayAuthorizationException`, a cancelled call becomes
+`OperationCanceledException`, and every other status becomes a base
+`MxGatewayException`. `MxGatewayException.StatusCode` carries the originating
+gRPC `Grpc.Core.StatusCode` (non-null whenever the failure came from a gRPC
+status), so callers can distinguish a transient outage (`Unavailable`) from a
+permanent error (`InvalidArgument`, `NotFound`) without downcasting
+`InnerException`.
+
 ## CLI Usage
 
 The test CLI supports deterministic JSON output for automation: