Resolve Client.Dotnet-004..008 code-review findings

Client.Dotnet-004: documented DefaultCallTimeout as both the per-attempt
deadline and the shared retry budget, and removed DeadlineExceeded from the
transient-retry set (a client-imposed deadline cannot be helped by retrying).

Client.Dotnet-005: RegisterAsync/AddItemAsync/AddItem2Async silently returned
0 when a successful reply lacked the typed payload. They now throw a
descriptive MxGatewayException.

Client.Dotnet-006: added XML docs to the previously undocumented public
members MaxGrpcMessageBytes, GatewayProtocolVersion, WorkerProtocolVersion.

Client.Dotnet-007: corrected the AcknowledgeAlarmAsync XML comment — the RPC
requires the admin scope, not a non-existent invoke:alarm-ack sub-scope.

Client.Dotnet-008: the CLI redactor missed env-var-sourced keys because the
caller passed only the --api-key option. Redaction now uses the same
resolver, stripping env-var keys too.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

clients/dotnet/MxGateway.Client/MxGatewaySession.cs

@@ -101,7 +101,8 @@ public sealed class MxGatewaySession : IAsyncDisposable
         MxCommandReply reply = await RegisterRawAsync(clientName, cancellationToken)
             .ConfigureAwait(false);
         reply.EnsureProtocolSuccess().EnsureMxAccessSuccess();
-        return reply.Register?.ServerHandle ?? reply.ReturnValue.Int32Value;
+        return reply.Register?.ServerHandle
+            ?? throw CreateMissingPayloadException(reply, "register");
     }
 
     /// <summary>
@@ -143,7 +144,8 @@ public sealed class MxGatewaySession : IAsyncDisposable
                 cancellationToken)
             .ConfigureAwait(false);
         reply.EnsureProtocolSuccess().EnsureMxAccessSuccess();
-        return reply.AddItem?.ItemHandle ?? reply.ReturnValue.Int32Value;
+        return reply.AddItem?.ItemHandle
+            ?? throw CreateMissingPayloadException(reply, "add_item");
     }
 
     /// <summary>
@@ -194,7 +196,8 @@ public sealed class MxGatewaySession : IAsyncDisposable
                 cancellationToken)
             .ConfigureAwait(false);
         reply.EnsureProtocolSuccess().EnsureMxAccessSuccess();
-        return reply.AddItem2?.ItemHandle ?? reply.ReturnValue.Int32Value;
+        return reply.AddItem2?.ItemHandle
+            ?? throw CreateMissingPayloadException(reply, "add_item2");
     }
 
     /// <summary>
@@ -723,4 +726,21 @@ public sealed class MxGatewaySession : IAsyncDisposable
             cancellationToken);
     }
 
+    /// <summary>
+    /// Builds the exception thrown when a command reply passed protocol and
+    /// MXAccess success checks but is missing the typed handle-bearing payload
+    /// the command contract requires. Surfacing this as a clear error avoids
+    /// silently handing a zero handle to the caller (it would otherwise fall
+    /// through to <see cref="MxCommandReply.ReturnValue"/>, which is 0 when the
+    /// reply carries no return value).
+    /// </summary>
+    private static MxGatewayException CreateMissingPayloadException(
+        MxCommandReply reply,
+        string expectedPayload)
+    {
+        return new MxGatewayException(
+            $"Gateway reply for command kind={reply.Kind} reported success but is missing "
+            + $"the required '{expectedPayload}' payload; cannot resolve a handle. "
+            + $"session={reply.SessionId}; correlation={reply.CorrelationId}");
+    }
 }