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/python/src/zb_mom_ww_mxgateway/session.py

@@ -919,19 +919,47 @@ def _value_secrets(value: MxValueInput) -> list[str]:
 
 
 def _redact_error(error: MxGatewayError, secrets: Sequence[str | None]) -> None:
-    """Scrub secret substrings from a raised error's message in place.
+    """Scrub secret substrings from a raised error's message and reply in place.
 
     Rewrites ``error.args[0]`` (the message returned by ``str(error)``) through
     the shared :func:`~zb_mom_ww_mxgateway.auth.redact_secret` seam so credential
-    text can never reach logs or be re-raised to a caller. The
-    ``protocol_status`` / ``raw_reply`` context is left untouched — those hold the
-    gateway's own fields, which never echo the client-supplied secret.
+    text can never reach logs or be re-raised to a caller.
+
+    A misbehaving MXAccess provider can echo the client-supplied credential back
+    verbatim in its failure diagnostics, so ``error.raw_reply`` (the protobuf
+    reply) can carry the secret in ``protocol_status.message``,
+    ``diagnostic_message``, and each ``statuses[].diagnostic_text``. A logger
+    dumping those structured fields would reintroduce the leak the message scrub
+    closes. When there is a secret to scrub and a reply is attached, this rebinds
+    ``error.raw_reply`` to a scrubbed deep copy so the raised exception carries no
+    credential text on any surface. The clone leaves the original reply untouched.
     """
     scrubbed = [secret for secret in secrets if secret]
     if not scrubbed:
         return
     if error.args and isinstance(error.args[0], str):
         error.args = (redact_secret(error.args[0], scrubbed), *error.args[1:])
+    if error.raw_reply is not None:
+        error.raw_reply = _redact_reply(error.raw_reply, scrubbed)
+
+
+def _redact_reply(reply: pb.MxCommandReply, secrets: Sequence[str]) -> pb.MxCommandReply:
+    """Return a deep copy of *reply* with credential text scrubbed from diagnostics.
+
+    Operates on a clone so the caller's original reply object is never mutated.
+    Only the free-text diagnostic fields that can echo a client-supplied secret
+    are scrubbed; the structured/enum fields the gateway itself sets are left as-is.
+    """
+    clone = type(reply)()
+    clone.CopyFrom(reply)
+    if clone.protocol_status.message:
+        clone.protocol_status.message = redact_secret(clone.protocol_status.message, secrets)
+    if clone.diagnostic_message:
+        clone.diagnostic_message = redact_secret(clone.diagnostic_message, secrets)
+    for status in clone.statuses:
+        if status.diagnostic_text:
+            status.diagnostic_text = redact_secret(status.diagnostic_text, secrets)
+    return clone
 
 
 from .client import GatewayClient  # noqa: E402