fix(telemetry): use safeJsonStringify in FileExporter to avoid circular reference crash

When --telemetry-outfile is configured, FileSpanExporter.serialize called
JSON.stringify directly on OTel ReadableSpan instances. The spans hold a
back-reference to BatchSpanProcessor (._shutdownOnce -> BindOnceFuture._that
-> BatchSpanProcessor), which forms a cycle and triggers
"TypeError: Converting circular structure to JSON" on every export. Combined
with DiagConsoleLogger, the error was repeatedly printed to stderr and
polluted the Ink TUI.

Switch FileExporter.serialize to the existing safeJsonStringify utility,
matching the upstream gemini-cli fix so future merges stay clean. Add a
focused regression test that mimics the BatchSpanProcessor cycle shape;
broader cycle behavior is already covered by safeJsonStringify.test.ts.

packages/core/src/telemetry/file-exporters.test.ts

@@ -0,0 +1,50 @@
+/**
+ * @license
+ * Copyright 2026 Qwen Team
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as fs from 'node:fs';
+import * as os from 'node:os';
+import * as path from 'node:path';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { FileSpanExporter } from './file-exporters.js';
+
+type SerializeAccess = { serialize: (data: unknown) => string };
+
+describe('FileExporter.serialize', () => {
+  let tmpDir: string;
+  let exporter: FileSpanExporter;
+  let serialize: (data: unknown) => string;
+
+  beforeEach(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'file-exporters-test-'));
+    exporter = new FileSpanExporter(path.join(tmpDir, 'out.jsonl'));
+    serialize = (exporter as unknown as SerializeAccess).serialize.bind(
+      exporter,
+    );
+  });
+
+  afterEach(async () => {
+    await exporter.shutdown();
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  });
+
+  // Regression for upstream PR #4689: a raw JSON.stringify on a ReadableSpan
+  // crashed because BatchSpanProcessor._shutdownOnce -> BindOnceFuture._that
+  // forms a cycle. The exporter must delegate to safeJsonStringify so cycles
+  // become "[Circular]" instead of throwing.
+  it('does not throw on BatchSpanProcessor-shaped cycle', () => {
+    const proc: Record<string, unknown> = { kind: 'BatchSpanProcessor' };
+    const future: Record<string, unknown> = { kind: 'BindOnceFuture' };
+    proc['_shutdownOnce'] = future;
+    future['_that'] = proc;
+    const span = { name: 'span-1', _spanProcessor: proc };
+
+    expect(() => serialize(span)).not.toThrow();
+    const out = serialize(span);
+    expect(out).toContain('"name": "span-1"');
+    expect(out).toContain('"[Circular]"');
+    expect(out.endsWith('\n')).toBe(true);
+  });
+});

packages/core/src/telemetry/file-exporters.ts

@@ -17,6 +17,7 @@ import type {
   PushMetricExporter,
 } from '@opentelemetry/sdk-metrics';
 import { AggregationTemporality } from '@opentelemetry/sdk-metrics';
+import { safeJsonStringify } from '../utils/safeJsonStringify.js';
 
 class FileExporter {
   protected writeStream: fs.WriteStream;
@@ -26,7 +27,7 @@ class FileExporter {
   }
 
   protected serialize(data: unknown): string {
-    return JSON.stringify(data, null, 2) + '\n';
+    return safeJsonStringify(data, 2) + '\n';
   }
 
   shutdown(): Promise<void> {