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/README.md

@@ -142,7 +142,8 @@ dotnet run --project clients/dotnet/MxGateway.Client.Cli -- smoke --endpoint htt
 `smoke` opens a session, registers a client, adds one item, advises it,
 optionally writes a value when `--type` and `--value` are supplied, reads a
 bounded event stream, and closes the session in a `finally` block. CLI error
-output redacts API keys supplied through `--api-key`.
+output redacts the effective API key, whether it was supplied through
+`--api-key` or resolved from the `--api-key-env` environment variable.
 
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