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/GrpcGalaxyRepositoryClientTransport.cs

@@ -36,7 +36,7 @@ internal sealed class GrpcGalaxyRepositoryClientTransport(
         }
         catch (RpcException exception)
         {
-            throw MapRpcException(exception, callOptions.CancellationToken);
+            throw RpcExceptionMapper.Map(exception, callOptions.CancellationToken);
         }
     }
 
@@ -53,7 +53,7 @@ internal sealed class GrpcGalaxyRepositoryClientTransport(
         }
         catch (RpcException exception)
         {
-            throw MapRpcException(exception, callOptions.CancellationToken);
+            throw RpcExceptionMapper.Map(exception, callOptions.CancellationToken);
         }
     }
 
@@ -70,7 +70,7 @@ internal sealed class GrpcGalaxyRepositoryClientTransport(
         }
         catch (RpcException exception)
         {
-            throw MapRpcException(exception, callOptions.CancellationToken);
+            throw RpcExceptionMapper.Map(exception, callOptions.CancellationToken);
         }
     }
 
@@ -101,7 +101,7 @@ internal sealed class GrpcGalaxyRepositoryClientTransport(
             }
             catch (RpcException exception)
             {
-                throw MapRpcException(exception, effectiveCancellationToken);
+                throw RpcExceptionMapper.Map(exception, effectiveCancellationToken);
             }
 
             yield return deployEvent;
@@ -115,28 +115,4 @@ internal sealed class GrpcGalaxyRepositoryClientTransport(
     {
         return WatchDeployEventsAsync(request, callOptions);
     }
-
-    private static Exception MapRpcException(
-        RpcException exception,
-        CancellationToken cancellationToken)
-    {
-        if (cancellationToken.IsCancellationRequested || exception.StatusCode == StatusCode.Cancelled)
-        {
-            return new OperationCanceledException(
-                exception.Status.Detail,
-                exception,
-                cancellationToken);
-        }
-
-        return exception.StatusCode switch
-        {
-            StatusCode.Unauthenticated => new MxGatewayAuthenticationException(
-                exception.Status.Detail,
-                innerException: exception),
-            StatusCode.PermissionDenied => new MxGatewayAuthorizationException(
-                exception.Status.Detail,
-                innerException: exception),
-            _ => new MxGatewayException(exception.Status.Detail, exception),
-        };
-    }
 }