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/MxGatewayClient.cs

@@ -184,9 +184,10 @@ public sealed class MxGatewayClient : IAsyncDisposable
 
     /// <summary>
     /// Acknowledges an active MXAccess alarm condition through the gateway. The
-    /// gateway authenticates the request against the API key's <c>invoke:alarm-ack</c>
-    /// scope and forwards the acknowledge to the worker's MXAccess session;
-    /// the resulting <see cref="MxStatusProxy"/> is returned in the reply.
+    /// gateway authorizes <see cref="AcknowledgeAlarmRequest"/> against the API
+    /// key's <c>admin</c> scope (there is no finer-grained alarm-ack sub-scope)
+    /// and forwards the acknowledge to the worker's MXAccess session; the
+    /// resulting <see cref="MxStatusProxy"/> is returned in the reply.
     /// </summary>
     /// <param name="request">The acknowledge request — alarm reference, comment, operator user.</param>
     /// <param name="cancellationToken">Cancellation token for the operation.</param>