fix(CLI-40): scrub the credential from the redacted error's structured reply, route MXACCESS_FAILURE to MxAccess (Rust), fix Go Subscribe terminal-error drop

Code-review follow-up on the CLI-40/41/44 branch.

ISSUE 1 (all five, critical): the message-only scrub still leaked the
server-echoed credential through the redacted error's structured reply accessor
(.NET Reply/Statuses, Java reply()/protocolStatus(), Go MxAccessError.Reply via
errors.As, Rust reply()/into_reply(), Python raw_reply). The redacted error now
carries a scrubbed clone of the reply (protocol_status.message,
diagnostic_message, statuses[].diagnostic_text), with per-language tests asserting
the reply accessor no longer contains the credential.

ISSUE 2 (Rust, critical): ensure_command_success routed MXACCESS_FAILURE to
Error::Command (unlike the other four clients), bypassing attach_secrets and
leaking via derived Debug/Display. MXACCESS_FAILURE now routes to Error::MxAccess,
fixing the cross-client inconsistency.

ISSUE 3 (Go, important): the CLI-44 terminal send was unconditionally
non-blocking, dropping a genuine terminal error under a full buffer on the
never-drop SubscribeEvents path. It is now reserved-slot-non-blocking only for the
cancel-on-overflow path and blocking for the never-drop path.

New shared fixture authenticate-user.echoed-credential-mxaccess-failure.reply.json
wired into all five suites. Minors: whitespace-secret guard on .NET/Java redact
helpers; Java preserves exception subtype on redaction; redaction-helper unit
tests (Go/Java/.NET). Docs (ClientBehaviorFixtures.md, ClientLibrariesDesign.md)
updated to make the structured-field claim true.

clients/dotnet/ZB.MOM.WW.MxGateway.Client.Tests/MxGatewaySessionReplyContractTests.cs

@@ -32,6 +32,44 @@ public sealed class MxGatewaySessionReplyContractTests
         Assert.DoesNotContain(password, exception.ToString(), StringComparison.Ordinal);
     }
 
+    /// <summary>
+    /// CLI-40: the redacted exception must not leak the echoed credential through any structured
+    /// accessor either — <see cref="MxAccessException.Reply"/> (protocol message, diagnostic
+    /// message, and each MXSTATUS_PROXY diagnostic text) and <see cref="MxGatewayException.Statuses"/>
+    /// all carry the server-echoed credential verbatim before the fix. Both the OK+negative-HRESULT
+    /// and the MXACCESS_FAILURE reply route to <see cref="MxAccessException"/>, so both must scrub.
+    /// </summary>
+    /// <param name="fixture">The echoed-credential reply fixture to drive.</param>
+    [Theory]
+    [InlineData("authenticate-user.echoed-credential.reply.json")]
+    [InlineData("authenticate-user.echoed-credential-mxaccess-failure.reply.json")]
+    public async Task AuthenticateUserAsync_RedactsEchoedCredentialInStructuredAccessors(string fixture)
+    {
+        const string password = "sup3rSecretVerify9f3a2b";
+        FakeGatewayTransport transport = CreateTransport();
+        transport.AddInvokeReply(ReadReplyFixture(fixture));
+        await using MxGatewayClient client = CreateClient(transport);
+        MxGatewaySession session = await client.OpenSessionAsync();
+
+        MxAccessException exception = await Assert.ThrowsAsync<MxAccessException>(
+            async () => await session.AuthenticateUserAsync(12, "operator", password));
+
+        Assert.DoesNotContain(password, exception.Message, StringComparison.Ordinal);
+        Assert.Contains("<redacted>", exception.Message, StringComparison.Ordinal);
+        Assert.DoesNotContain(password, exception.ToString(), StringComparison.Ordinal);
+        Assert.DoesNotContain(password, exception.Reply.ProtocolStatus.Message, StringComparison.Ordinal);
+        Assert.DoesNotContain(password, exception.Reply.DiagnosticMessage, StringComparison.Ordinal);
+        foreach (MxStatusProxy status in exception.Reply.Statuses)
+        {
+            Assert.DoesNotContain(password, status.DiagnosticText, StringComparison.Ordinal);
+        }
+
+        foreach (MxStatusProxy status in exception.Statuses)
+        {
+            Assert.DoesNotContain(password, status.DiagnosticText, StringComparison.Ordinal);
+        }
+    }
+
     /// <summary>
     /// CLI-41: an OK reply that carries neither the typed AuthenticateUser payload nor an
     /// int32 return_value is a malformed reply, surfaced as a typed exception rather than an NRE.