fix(#838): validate complete broker reply frames

docs/scratchpads/838-broker-acceptance-flake.md

@@ -36,3 +36,7 @@ Eliminate high-contention uncaught JSON parse failures in lease-broker and mutat
 - Residual/tripwire: an unavoidable peer disconnect or send failure can still lose acknowledgement after a valid transaction commits. The affected adapter call fails closed. A valid `promote_lease` may nevertheless remain VERIFIED after its acknowledgement is lost; that is authority-observability divergence requiring WI-3/Opus security review rather than expansion of #838. #838 does not add retries or attempt a protocol redesign.
 - Final package evidence: recursive Mosaic dependency build passed; 73 Vitest files / 1,392 tests passed; deadline unit tests 2/2, real runtime tool tests 15/15, launch guard tests 12/12, inventory 14/14, and shell regressions passed.
 - Final coverage: `broker-test-client.ts` 99.24% statements/lines, 86.11% branches, 100% functions under per-file >=85% thresholds. Repository typecheck 42/42, lint 23/23, and format check passed.
+- Independent Codex exact-head review requested one framing fix: a valid frame followed by trailing bytes in a later socket chunk could resolve before the garbage arrived. Security review also flagged complete token-bearing reply bodies in diagnostic properties/logs.
+- Review RED evidence: delayed cross-chunk garbage resolved `{ ok: true }` instead of rejecting, and a truncated `promotion_token` remained in the typed error. The tests use separate timed writes to prevent kernel/event-loop coalescing.
+- Review remediation: the shared client now accumulates through EOF, requires exactly one terminal newline, then parses inside a rejecting error boundary. Diagnostic bodies are capped at 256 bytes, sensitive broker fields are fully redacted, and a SHA-256 digest preserves correlation without credential disclosure.
+- Post-review coverage: `broker-test-client.ts` 99.35% statements/lines, 86.66% branches, 100% functions. Final full suite is 73 files / 1,394 tests plus all Python/shell gates; recursive build, typecheck, lint, and format are green.

packages/mosaic/src/lease-broker/broker-test-client.ts

@@ -1,7 +1,10 @@
+import { createHash } from 'node:crypto';
 import { createConnection, type Socket } from 'node:net';
 
 const DEFAULT_TIMEOUT_MS = 3_000;
 const MAX_REPLY_BYTES = 64 * 1024;
+const MAX_DIAGNOSTIC_BYTES = 256;
+const SENSITIVE_REPLY_FIELD = /"(?:promotion_token|session_id|token)"\s*:/;
 
 export interface BrokerTestClientOptions {
   timeoutMs?: number;
@@ -17,6 +20,8 @@ export class BrokerTransportError extends Error {
   public readonly responseLength: number;
   public readonly responseBytes: string;
   public readonly responseHex: string;
+  public readonly responsePreview: string;
+  public readonly responseSha256: string;
 
   public constructor(
     public readonly kind: BrokerTransportFailureKind,
@@ -24,18 +29,43 @@ export class BrokerTransportError extends Error {
     response: Buffer,
     public readonly attempts = 1,
   ) {
-    super(`${description}; attempts=${attempts}; ${responseContext(response)}`);
+    const diagnostics = responseDiagnostics(response);
+    super(`${description}; attempts=${attempts}; ${responseContext(response, diagnostics)}`);
     this.name = 'BrokerTransportError';
     this.responseLength = response.length;
-    this.responseBytes = response.toString('utf8');
-    this.responseHex = response.toString('hex');
+    this.responseBytes = diagnostics.preview;
+    this.responseHex = diagnostics.hex;
+    this.responsePreview = diagnostics.preview;
+    this.responseSha256 = diagnostics.sha256;
   }
 }
 
-function responseContext(response: Buffer): string {
-  const text = JSON.stringify(response.toString('utf8'));
-  const hex = response.length === 0 ? '<empty>' : response.toString('hex');
-  return `length=${response.length}; bytes=${text}; hex=${hex}`;
+interface ResponseDiagnostics {
+  preview: string;
+  hex: string;
+  sha256: string;
+}
+
+function responseDiagnostics(response: Buffer): ResponseDiagnostics {
+  const fullText = response.toString('utf8');
+  const sensitive = SENSITIVE_REPLY_FIELD.test(fullText);
+  const bounded = response.subarray(0, MAX_DIAGNOSTIC_BYTES);
+  const suffix = response.length > MAX_DIAGNOSTIC_BYTES ? '…' : '';
+  return {
+    preview:
+      response.length === 0
+        ? '<empty>'
+        : sensitive
+          ? '<redacted-sensitive-reply>'
+          : `${bounded.toString('utf8')}${suffix}`,
+    hex: sensitive ? '<redacted>' : `${bounded.toString('hex')}${suffix}`,
+    sha256: createHash('sha256').update(response).digest('hex'),
+  };
+}
+
+function responseContext(response: Buffer, diagnostics = responseDiagnostics(response)): string {
+  const hex = diagnostics.hex.length === 0 ? '<empty>' : diagnostics.hex;
+  return `length=${response.length}; bytes=${JSON.stringify(diagnostics.preview)}; hex=${hex}; sha256=${diagnostics.sha256}`;
 }
 
 function malformedReply(response: Buffer, reason: string): BrokerTransportError {
@@ -86,11 +116,22 @@ function readBrokerReplyAttempt<T extends object>(
       response = Buffer.concat([response, chunk]);
       if (response.length > MAX_REPLY_BYTES) {
         fail(malformedReply(response, `exceeds ${MAX_REPLY_BYTES} bytes`));
+      }
+    });
+    socket.once('end', () => {
+      if (settled) return;
+      const newline = response.indexOf(0x0a);
+      if (response.length === 0 || newline < 0) {
+        fail(
+          new BrokerTransportError(
+            'early-close',
+            'Broker reply socket closed before newline',
+            response,
+          ),
+        );
         return;
       }
 
-      const newline = response.indexOf(0x0a);
-      if (newline < 0) return;
       if (newline !== response.length - 1) {
         fail(malformedReply(response, 'contains bytes after the newline terminator'));
         return;
@@ -109,17 +150,6 @@ function readBrokerReplyAttempt<T extends object>(
         );
       }
     });
-    socket.once('end', () => {
-      if (!settled) {
-        fail(
-          new BrokerTransportError(
-            'early-close',
-            'Broker reply socket closed before newline',
-            response,
-          ),
-        );
-      }
-    });
     socket.once('connect', () => {
       try {
         write(socket);