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/rust/tests/client_behavior.rs

@@ -84,8 +84,10 @@ async fn session_helpers_build_commands_and_preserve_command_errors() {
         .write(12, 34, ClientMxValue::int32(123), 0)
         .await
         .unwrap_err();
-    let Error::Command(error) = error else {
-        panic!("write failure should preserve the raw command reply: {error:?}");
+    // A MXACCESS_FAILURE-coded reply is an MXAccess-level failure, routed to
+    // Error::MxAccess (matching .NET/Java/Go/Python) rather than Error::Command.
+    let Error::MxAccess(error) = error else {
+        panic!("MXACCESS_FAILURE reply should route to Error::MxAccess: {error:?}");
     };
     assert_eq!(
         error.reply().protocol_status.as_ref().unwrap().code,
@@ -841,6 +843,92 @@ async fn authenticate_user_scrubs_exact_caller_credential_echoed_in_diagnostic()
     );
 }
 
+/// Drive `authenticate_user` against a canned reply that echoes the caller's
+/// credential in every string field, then assert the surfaced
+/// [`Error::MxAccess`] leaks it nowhere — neither through the structured reply a
+/// caller can read back (`reply().protocol_status.message`,
+/// `reply().diagnostic_message`, `reply().statuses[i].diagnostic_text`) nor
+/// through `Display`/`Debug`.
+async fn assert_authenticate_user_scrubs_structured_reply(fixture: &str) {
+    let credential = "sup3rSecretVerify9f3a2b";
+    let state = Arc::new(FakeState::default());
+    *state.invoke_override.lock().await = Some(InvokeOverride::CannedReply(Box::new(
+        command_reply_fixture(fixture),
+    )));
+    let endpoint = spawn_fake_gateway(state.clone()).await;
+    let client = GatewayClient::connect(ClientOptions::new(endpoint))
+        .await
+        .unwrap();
+    let session = client.session("session-fixture");
+
+    let error = session
+        .authenticate_user(7, "verifier", credential)
+        .await
+        .unwrap_err();
+
+    let Error::MxAccess(mx_access) = &error else {
+        panic!("{fixture}: credential-echoed reply must route to Error::MxAccess, got {error:?}");
+    };
+
+    // The structured reply a caller can read back must be scrubbed too — the raw
+    // MxCommandReply otherwise reintroduces the leak Display/Debug already close.
+    let reply = mx_access.reply();
+    if let Some(status) = reply.protocol_status.as_ref() {
+        assert!(
+            !status.message.contains(credential),
+            "{fixture}: credential leaked via reply().protocol_status.message: {}",
+            status.message
+        );
+    }
+    assert!(
+        !reply.diagnostic_message.contains(credential),
+        "{fixture}: credential leaked via reply().diagnostic_message: {}",
+        reply.diagnostic_message
+    );
+    for (index, status) in reply.statuses.iter().enumerate() {
+        assert!(
+            !status.diagnostic_text.contains(credential),
+            "{fixture}: credential leaked via reply().statuses[{index}].diagnostic_text: {}",
+            status.diagnostic_text
+        );
+    }
+
+    let display = error.to_string();
+    let debug = format!("{error:?}");
+    assert!(
+        !display.contains(credential),
+        "{fixture}: credential leaked into Display: {display}"
+    );
+    assert!(
+        !debug.contains(credential),
+        "{fixture}: credential leaked into Debug: {debug}"
+    );
+    assert!(
+        display.contains("<redacted>"),
+        "{fixture}: Display must mark the redaction: {display}"
+    );
+}
+
+#[tokio::test]
+async fn authenticate_user_scrubs_credential_from_structured_reply_ok_protocol_variant() {
+    // OK protocol envelope + negative hresult: already Error::MxAccess before
+    // ISSUE 2, but the stored reply's string fields still leaked the credential.
+    assert_authenticate_user_scrubs_structured_reply(
+        "authenticate-user.echoed-credential.reply.json",
+    )
+    .await;
+}
+
+#[tokio::test]
+async fn authenticate_user_scrubs_credential_from_structured_reply_mxaccess_failure_variant() {
+    // PROTOCOL_STATUS_CODE_MXACCESS_FAILURE: before ISSUE 2 this landed in
+    // Error::Command (unscrubbed, raw Display/Debug) — the red-first case.
+    assert_authenticate_user_scrubs_structured_reply(
+        "authenticate-user.echoed-credential-mxaccess-failure.reply.json",
+    )
+    .await;
+}
+
 #[tokio::test]
 async fn authenticate_user_maps_missing_payload_reply_to_malformed_reply() {
     // CLI-41: an OK reply with neither a typed AuthenticateUser payload nor a
@@ -1548,15 +1636,35 @@ fn command_reply_fixture(file_name: &str) -> MxCommandReply {
         })
     });
 
+    // Honor the fixture's real protocol status (code + message) so a canned
+    // reply can drive the MXACCESS_FAILURE routing path, not just an OK
+    // envelope. Falls back to an OK envelope when the fixture omits it.
+    let protocol_status = fixture.get("protocolStatus").map_or_else(
+        || ok_status("command ok"),
+        |status| {
+            let code_name = status["code"].as_str().unwrap_or("PROTOCOL_STATUS_CODE_OK");
+            ProtocolStatus {
+                code: ProtocolStatusCode::from_str_name(code_name)
+                    .unwrap_or_else(|| panic!("unknown protocol status code {code_name}"))
+                    as i32,
+                message: status["message"].as_str().unwrap_or_default().to_owned(),
+            }
+        },
+    );
+
     MxCommandReply {
         session_id: fixture["sessionId"].as_str().unwrap_or_default().to_owned(),
         correlation_id: fixture["correlationId"]
             .as_str()
             .unwrap_or_default()
             .to_owned(),
-        protocol_status: Some(ok_status("command ok")),
+        protocol_status: Some(protocol_status),
         hresult: fixture["hresult"].as_i64().map(|hresult| hresult as i32),
         statuses,
+        diagnostic_message: fixture["diagnosticMessage"]
+            .as_str()
+            .unwrap_or_default()
+            .to_owned(),
         return_value,
         ..MxCommandReply::default()
     }