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/proto/fixtures/behavior/manifest.json

@@ -48,6 +48,27 @@
       "path": "command-replies/write.hresult-e-fail.reply.json",
       "expectation": "A negative HRESULT fails the reply even when every status entry reports MX_STATUS_CATEGORY_OK."
     },
+    {
+      "id": "command-reply.authenticate-user.echoed-credential",
+      "category": "command_replies",
+      "messageType": "mxaccess_gateway.v1.MxCommandReply",
+      "path": "command-replies/authenticate-user.echoed-credential.reply.json",
+      "expectation": "When a gateway/MXAccess diagnostic echoes the caller's credential back, the surfaced error redacts the exact secret and never leaks the verbatim value."
+    },
+    {
+      "id": "command-reply.authenticate-user.missing-payload",
+      "category": "command_replies",
+      "messageType": "mxaccess_gateway.v1.MxCommandReply",
+      "path": "command-replies/authenticate-user.missing-payload.reply.json",
+      "expectation": "An OK reply with neither the typed AuthenticateUser payload nor a return_value raises a typed malformed-reply error, never a proto3 default 0 and never an NRE."
+    },
+    {
+      "id": "command-reply.authenticate-user.return-value-only",
+      "category": "command_replies",
+      "messageType": "mxaccess_gateway.v1.MxCommandReply",
+      "path": "command-replies/authenticate-user.return-value-only.reply.json",
+      "expectation": "An OK reply missing the typed AuthenticateUser payload but carrying an int32 return_value falls back to the return_value (legacy-worker compatibility)."
+    },
     {
       "id": "event-stream.session-ordered",
       "category": "event_streams",