fix(agents): keep failover detail truncation UTF-16 safe (#104487)

* fix(agents): keep failover detail truncation UTF-16 safe

Replace raw .slice(0, MAX_FAILOVER_DETAIL_CHARS) with truncateUtf16Safe
so provider error details containing emoji or other non-BMP characters
are not split at a surrogate-pair boundary before failover classification.

Adds a regression test via extractFailoverSignalDetails.

* test(agents): tighten failover UTF-16 coverage

---------

Co-authored-by: chengzhichao-xydt <chengzhichao-xydt@users.noreply.github.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/agents/embedded-agent-helpers/errors.test.ts

@@ -4,7 +4,11 @@ import { beforeEach, describe, expect, it, vi } from "vitest";
 import type { OpenClawConfig } from "../../config/types.openclaw.js";
 import { MALFORMED_STREAMING_FRAGMENT_ERROR_MESSAGE } from "../../shared/assistant-error-format.js";
 import { makeAssistantMessageFixture } from "../test-helpers/assistant-message-fixtures.js";
-import { formatAssistantErrorText, isLikelyContextOverflowError } from "./errors.js";
+import {
+  extractFailoverSignalDetails,
+  formatAssistantErrorText,
+  isLikelyContextOverflowError,
+} from "./errors.js";
 
 const { toolPolicyAuditInfo } = vi.hoisted(() => ({
   toolPolicyAuditInfo: vi.fn(),
@@ -98,6 +102,25 @@ describe("formatAssistantErrorText streaming JSON parse classification", () => {
   });
 });
 
+describe("extractFailoverSignalDetails", () => {
+  it.each([
+    {
+      name: "backs off before a split surrogate pair",
+      input: `${"a".repeat(999)}🎉!`,
+      expected: "a".repeat(999),
+    },
+    {
+      name: "keeps the full ASCII budget",
+      input: "a".repeat(1001),
+      expected: "a".repeat(1000),
+    },
+  ])("$name in nested provider details", ({ input, expected }) => {
+    const details = extractFailoverSignalDetails({ error: { body: { detail: input } } });
+
+    expect(details).toEqual([expected]);
+  });
+});
+
 describe("isLikelyContextOverflowError", () => {
   it("detects Codex promptError wording for a full context window", () => {
     expect(

src/agents/embedded-agent-helpers/errors.ts

@@ -308,7 +308,7 @@ function normalizeFailoverDetailString(value: string | undefined): string | unde
     return undefined;
   }
   return trimmed.length > MAX_FAILOVER_DETAIL_CHARS
-    ? trimmed.slice(0, MAX_FAILOVER_DETAIL_CHARS)
+    ? truncateUtf16Safe(trimmed, MAX_FAILOVER_DETAIL_CHARS)
     : trimmed;
 }