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/MxGatewayClientContractInfo.cs

@@ -7,9 +7,19 @@ namespace MxGateway.Client;
 /// </summary>
 public static class MxGatewayClientContractInfo
 {
+    /// <summary>
+    /// Gets the gateway gRPC protocol version compiled into this client package.
+    /// A client and gateway are wire-compatible only when this value matches the
+    /// gateway's advertised gateway protocol version.
+    /// </summary>
     public const uint GatewayProtocolVersion =
         GatewayContractInfo.GatewayProtocolVersion;
 
+    /// <summary>
+    /// Gets the worker frame protocol version compiled into this client package.
+    /// Exposed for diagnostics so callers can report the worker protocol the
+    /// shared contracts were generated against.
+    /// </summary>
     public const uint WorkerProtocolVersion =
         GatewayContractInfo.WorkerProtocolVersion;
 }