feat(FN-4636): complete Step 2 — add native sandbox backend

Fusion-Task-Id: FN-4636
Fusion-Task-Lineage: 38ff2f48-4cb1-42c2-8f64-eb3b8d3d7f2c

packages/engine/src/sandbox/__tests__/native.test.ts

@@ -0,0 +1,72 @@
+import { cwd } from "node:process";
+
+import { describe, expect, it } from "vitest";
+
+import { NativeSandboxBackend } from "../native.js";
+
+describe("NativeSandboxBackend", () => {
+  it("returns stdout on success", async () => {
+    const backend = new NativeSandboxBackend();
+    const result = await backend.run("node -e 'process.stdout.write(\"ok\")'", {
+      cwd: cwd(),
+      timeoutMs: 5_000,
+      maxBuffer: 1024 * 1024,
+      encoding: "utf-8",
+    });
+
+    expect(result.exitCode).toBe(0);
+    expect(result.stdout).toBe("ok");
+    expect(result.timedOut).toBe(false);
+    expect(result.bufferExceeded).toBe(false);
+  });
+
+  it("maps timeout failures", async () => {
+    const backend = new NativeSandboxBackend();
+    const result = await backend.run("node -e 'setTimeout(() => {}, 1000)'", {
+      cwd: cwd(),
+      timeoutMs: 50,
+      maxBuffer: 1024 * 1024,
+      encoding: "utf-8",
+    });
+
+    expect(result.exitCode).toBeNull();
+    expect(result.timedOut).toBe(true);
+    expect(result.signal).toBe("SIGTERM");
+  });
+
+  it("maps non-zero exits", async () => {
+    const backend = new NativeSandboxBackend();
+    const result = await backend.run("node -e 'process.stderr.write(\"fail\"); process.exit(7)'", {
+      cwd: cwd(),
+      timeoutMs: 5_000,
+      maxBuffer: 1024 * 1024,
+      encoding: "utf-8",
+    });
+
+    expect(result.exitCode).toBe(7);
+    expect(result.stderr).toContain("fail");
+    expect(result.timedOut).toBe(false);
+  });
+
+  it("maps maxBuffer failures", async () => {
+    const backend = new NativeSandboxBackend();
+    const result = await backend.run("node -e 'process.stdout.write(\"x\".repeat(5000))'", {
+      cwd: cwd(),
+      timeoutMs: 5_000,
+      maxBuffer: 512,
+      encoding: "utf-8",
+    });
+
+    expect(result.bufferExceeded).toBe(true);
+    expect(result.exitCode).toBeNull();
+  });
+
+  it("prepare/dispose are idempotent no-ops", async () => {
+    const backend = new NativeSandboxBackend();
+
+    await expect(backend.prepare({ allowNetwork: true })).resolves.toBeUndefined();
+    await expect(backend.prepare({ allowNetwork: false })).resolves.toBeUndefined();
+    await expect(backend.dispose()).resolves.toBeUndefined();
+    await expect(backend.dispose()).resolves.toBeUndefined();
+  });
+});