feat(FN-4636): complete Step 3 — add sandbox backend resolver

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

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

@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+
+import { NativeSandboxBackend } from "../native.js";
+import { resolveSandboxBackend } from "../index.js";
+
+describe("resolveSandboxBackend", () => {
+  it("returns native for undefined backend", () => {
+    expect(resolveSandboxBackend()).toBeInstanceOf(NativeSandboxBackend);
+  });
+
+  it("returns native for explicit native backend", () => {
+    expect(resolveSandboxBackend({ backendId: "native" })).toBeInstanceOf(NativeSandboxBackend);
+  });
+
+  it("returns native for unknown backend id", () => {
+    expect(resolveSandboxBackend({ backendId: "bubblewrap" })).toBeInstanceOf(NativeSandboxBackend);
+  });
+});

packages/engine/src/sandbox/index.ts

@@ -0,0 +1,29 @@
+import { NativeSandboxBackend } from "./native.js";
+import type { SandboxBackend, SandboxCapabilities } from "./types.js";
+
+export type {
+  SandboxBackend,
+  SandboxCapabilities,
+  SandboxPolicy,
+  SandboxRunOptions,
+  SandboxRunResult,
+} from "./types.js";
+
+let sandboxBackendOverrideForTests: SandboxBackend | null = null;
+
+export function __setSandboxBackendForTests(backend: SandboxBackend | null): void {
+  sandboxBackendOverrideForTests = backend;
+}
+
+export function __resetSandboxBackendForTests(): void {
+  sandboxBackendOverrideForTests = null;
+}
+
+export function resolveSandboxBackend(_options?: { backendId?: SandboxCapabilities["id"] }): SandboxBackend {
+  if (sandboxBackendOverrideForTests) {
+    return sandboxBackendOverrideForTests;
+  }
+
+  // TODO(FN-4637/FN-4638/FN-4642): branch by backend id once additional implementations land.
+  return new NativeSandboxBackend();
+}