fix: harden config prototype-key guards (#22968) (thanks @Clawborn)

src/config/legacy.shared.test.ts

@@ -0,0 +1,23 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { mergeMissing } from "./legacy.shared.js";
+
+describe("mergeMissing prototype pollution guard", () => {
+  afterEach(() => {
+    delete (Object.prototype as Record<string, unknown>).polluted;
+  });
+
+  it("ignores __proto__ keys without polluting Object.prototype", () => {
+    const target = { safe: { keep: true } } as Record<string, unknown>;
+    const source = JSON.parse('{"safe":{"next":1},"__proto__":{"polluted":true}}') as Record<
+      string,
+      unknown
+    >;
+
+    mergeMissing(target, source);
+
+    expect((target.safe as Record<string, unknown>).keep).toBe(true);
+    expect((target.safe as Record<string, unknown>).next).toBe(1);
+    expect(target.polluted).toBeUndefined();
+    expect((Object.prototype as Record<string, unknown>).polluted).toBeUndefined();
+  });
+});