fix(agents): truncate console text on code-point boundaries (#96296)

sanitizeForConsole filtered control characters code-point-aware but then
truncated with sanitized.slice(0, maxChars), which cuts on UTF-16 code units.
When the cap landed between the two code units of an astral character (emoji,
CJK extension, etc.) the output ended in a lone high surrogate before the
ellipsis. Cap on code points instead.

Co-authored-by: ly-wang19 <ly-wang19@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/agents/console-sanitize.test.ts

@@ -0,0 +1,20 @@
+// Console sanitizer tests cover control-char filtering and code-point-safe truncation.
+import { describe, expect, it } from "vitest";
+import { sanitizeForConsole } from "./console-sanitize.js";
+
+const hasLoneSurrogate = (value: string) =>
+  /[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]/.test(value);
+
+describe("sanitizeForConsole", () => {
+  it("truncates on code-point boundaries without splitting a surrogate pair", () => {
+    const grin = String.fromCodePoint(0x1f600); // 😀 — two UTF-16 code units
+    const out = sanitizeForConsole(grin.repeat(6), 3);
+    expect(out).toBe(`${grin.repeat(3)}…`);
+    expect(out !== undefined && hasLoneSurrogate(out)).toBe(false);
+  });
+
+  it("filters control chars, flattens whitespace, and leaves short strings intact", () => {
+    expect(sanitizeForConsole("  hello\tworld  ")).toBe("hello world");
+    expect(sanitizeForConsole(undefined)).toBeUndefined();
+  });
+});

src/agents/console-sanitize.ts

@@ -22,5 +22,11 @@ export function sanitizeForConsole(text: string | undefined, maxChars = 200): st
     .replace(/[\r\n\t]+/g, " ")
     .replace(/\s+/g, " ")
     .trim();
-  return sanitized.length > maxChars ? `${sanitized.slice(0, maxChars)}…` : sanitized;
+  const codePoints = Array.from(sanitized);
+  if (codePoints.length <= maxChars) {
+    return sanitized;
+  }
+  // Cap on code-point boundaries so a maxChars cut never splits a surrogate pair (emoji/astral) and
+  // leaves a lone surrogate before the ellipsis.
+  return `${codePoints.slice(0, maxChars).join("")}…`;
 }