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/tests/test_malformed_reply.py

@@ -82,15 +82,31 @@ async def test_add_buffered_item_missing_payload_raises_malformed_reply() -> Non
     assert captured.value.raw_reply is reply
 
 
+@pytest.mark.parametrize(
+    "fixture",
+    [
+        "command-replies/authenticate-user.echoed-credential.reply.json",
+        "command-replies/authenticate-user.echoed-credential-mxaccess-failure.reply.json",
+    ],
+)
 @pytest.mark.asyncio
-async def test_authenticate_user_echoed_credential_is_scrubbed() -> None:
+async def test_authenticate_user_echoed_credential_is_scrubbed(fixture: str) -> None:
     credential = "sup3rSecretVerify9f3a2b"
-    reply = _load_reply("command-replies/authenticate-user.echoed-credential.reply.json")
+    reply = _load_reply(fixture)
     session, _ = await _session_with([reply])
 
     with pytest.raises(MxAccessError) as captured:
         await session.authenticate_user(12, "operator", credential)
 
-    message = str(captured.value)
+    exc = captured.value
+    message = str(exc)
     assert credential not in message
     assert "[redacted]" in message
+
+    # The credential must not survive in the structured protobuf context either:
+    # a logger dumping raw_reply's fields would otherwise reintroduce the leak.
+    assert exc.raw_reply is not None
+    assert credential not in exc.raw_reply.protocol_status.message
+    assert credential not in exc.raw_reply.diagnostic_message
+    for status in exc.raw_reply.statuses:
+        assert credential not in status.diagnostic_text