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/go/mxgateway/errors.go

@@ -72,6 +72,25 @@ func redactSecrets(err error, secrets ...string) error {
 // dropping events. Match it with errors.Is.
 var ErrSlowConsumer = errors.New("mxgateway: event consumer fell behind; stream terminated")
 
+// MalformedReplyError reports an OK command reply that carried neither the
+// typed payload the operation expected nor a usable int32 return_value, so the
+// client cannot produce a result. It gives every affected helper one uniform,
+// inspectable failure instead of silently returning a zero value.
+type MalformedReplyError struct {
+	// Op names the operation whose reply was malformed.
+	Op string
+	// Detail explains what the reply was missing.
+	Detail string
+}
+
+// Error returns the formatted malformed-reply message.
+func (e *MalformedReplyError) Error() string {
+	if e == nil {
+		return ""
+	}
+	return fmt.Sprintf("mxgateway: %s returned a malformed reply: %s", e.Op, e.Detail)
+}
+
 // GatewayError wraps transport-level gRPC failures.
 type GatewayError struct {
 	// Op names the operation that failed (for example "dial" or "invoke").