refactor(core): share JSON utf8 byte counting helper

src/infra/json-utf8-bytes.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, it } from "vitest";
+import { jsonUtf8Bytes } from "./json-utf8-bytes.js";
+
+describe("jsonUtf8Bytes", () => {
+  it("returns utf8 byte length for serializable values", () => {
+    expect(jsonUtf8Bytes({ a: "x", b: [1, 2, 3] })).toBe(
+      Buffer.byteLength(JSON.stringify({ a: "x", b: [1, 2, 3] }), "utf8"),
+    );
+  });
+
+  it("falls back to string conversion when JSON serialization throws", () => {
+    const circular: { self?: unknown } = {};
+    circular.self = circular;
+    expect(jsonUtf8Bytes(circular)).toBe(Buffer.byteLength("[object Object]", "utf8"));
+  });
+});

src/infra/json-utf8-bytes.ts

@@ -0,0 +1,7 @@
+export function jsonUtf8Bytes(value: unknown): number {
+  try {
+    return Buffer.byteLength(JSON.stringify(value), "utf8");
+  } catch {
+    return Buffer.byteLength(String(value), "utf8");
+  }
+}