fix(CLI-40,CLI-41,CLI-44): exact-secret scrub, uniform malformed-reply contract, Go terminal-error mislabel

CLI-40: port the exact-secret credential scrub to Rust/Java/.NET (Go/Python
already did it). AuthenticateUser/WriteSecured(2) helpers now redact the exact
caller-supplied secret from any surfaced error, as defense-in-depth on top of the
by-construction guarantee. Rust hand-writes a redacting Debug (derived Debug would
leak the reply); Java/.NET rebuild the same exception type with the redacted
message and do not carry the secret-bearing original forward (so ToString/stack
traces stay clean too).

CLI-41: uniform malformed-reply contract for AuthenticateUser/ArchestrAUserToId/
AddBufferedItem across all five clients — typed payload, else a present int32
return_value, else a typed malformed-reply error. Fixes Go/Java silent-0, .NET
NRE, and Rust's own internal inconsistency.

CLI-44: the Go event goroutine's Recv-error path now uses a non-blocking
sendTerminalEventResult on the reserved slot, so a genuine terminal stream error
is reported as itself instead of being mislabeled ErrSlowConsumer under overflow.

Riders from the CLI-37/38 review: (a) .NET ToDiagnosticSummary and Python
_mxaccess_message surface the raw success member (diagnostics-only parity with
Rust); (b) the status-conversion fixture carries an independent wantSuccess
boolean and the Go/.NET fixture tests assert against it instead of recomputing
the formula under test.

Shared fixtures (authenticate-user.{echoed-credential,missing-payload,
return-value-only}.reply.json) + manifest + ClientBehaviorFixtures.md +
ClientLibrariesDesign.md updated in the same change. Tracking: CLI-40/41/44 -> Done.

clients/python/src/zb_mom_ww_mxgateway/session.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 from collections.abc import AsyncIterator, Sequence
 
 from .auth import redact_secret
-from .errors import MxGatewayError, ensure_mxaccess_success
+from .errors import MalformedReplyError, MxGatewayError, ensure_mxaccess_success
 from .events import ReplayGap
 from .generated import mxaccess_gateway_pb2 as pb
 from .values import MxValueInput, to_mx_value
@@ -710,7 +710,15 @@ class Session:
             correlation_id=correlation_id,
             secrets=[verify_user_password],
         )
-        return reply.authenticate_user.user_id
+        if reply.HasField("authenticate_user"):
+            return reply.authenticate_user.user_id
+        if reply.HasField("return_value") and reply.return_value.WhichOneof("kind") == "int32_value":
+            return reply.return_value.int32_value
+        raise MalformedReplyError(
+            "authenticate_user returned a malformed reply: OK reply carried "
+            "neither the typed payload nor an int32 return_value",
+            raw_reply=reply,
+        )
 
     async def archestra_user_to_id(
         self,
@@ -730,7 +738,15 @@ class Session:
             ),
             correlation_id=correlation_id,
         )
-        return reply.archestra_user_to_id.user_id
+        if reply.HasField("archestra_user_to_id"):
+            return reply.archestra_user_to_id.user_id
+        if reply.HasField("return_value") and reply.return_value.WhichOneof("kind") == "int32_value":
+            return reply.return_value.int32_value
+        raise MalformedReplyError(
+            "archestra_user_to_id returned a malformed reply: OK reply carried "
+            "neither the typed payload nor an int32 return_value",
+            raw_reply=reply,
+        )
 
     async def add_buffered_item(
         self,
@@ -752,7 +768,15 @@ class Session:
             ),
             correlation_id=correlation_id,
         )
-        return reply.add_buffered_item.item_handle
+        if reply.HasField("add_buffered_item"):
+            return reply.add_buffered_item.item_handle
+        if reply.HasField("return_value") and reply.return_value.WhichOneof("kind") == "int32_value":
+            return reply.return_value.int32_value
+        raise MalformedReplyError(
+            "add_buffered_item returned a malformed reply: OK reply carried "
+            "neither the typed payload nor an int32 return_value",
+            raw_reply=reply,
+        )
 
     async def set_buffered_update_interval(
         self,