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/proto/fixtures/behavior/command-replies/authenticate-user.echoed-credential-mxaccess-failure.reply.json

@@ -0,0 +1,22 @@
+{
+  "sessionId": "session-fixture",
+  "correlationId": "gateway-correlation-authenticate-echoed-mxaccess-failure",
+  "kind": "MX_COMMAND_KIND_AUTHENTICATE_USER",
+  "protocolStatus": {
+    "code": "PROTOCOL_STATUS_CODE_MXACCESS_FAILURE",
+    "message": "MXAccess AuthenticateUser rejected credential 'sup3rSecretVerify9f3a2b'."
+  },
+  "hresult": -2147024891,
+  "statuses": [
+    {
+      "success": 0,
+      "category": "MX_STATUS_CATEGORY_SECURITY_ERROR",
+      "detectedBy": "MX_STATUS_SOURCE_RESPONDING_NMX",
+      "detail": 5,
+      "rawCategory": 8,
+      "rawDetectedBy": 5,
+      "diagnosticText": "Authentication failed for password 'sup3rSecretVerify9f3a2b'."
+    }
+  ],
+  "diagnosticMessage": "MXAccess echoed the credential 'sup3rSecretVerify9f3a2b' back in its failure diagnostic."
+}