fix(sdk/js): treat null volume backends as absent in validation

Address Codex review feedback: use `!= null` instead of `!== undefined`
so that JSON-normalized null optionals (e.g. `{ ossfs: null }`) are
correctly treated as absent rather than counting as a specified backend.

Add test case to verify null backends do not trigger multi-backend error.

Author: perhapzz

sdks/sandbox/javascript/src/sandbox.ts

@@ -246,7 +246,7 @@ export class Sandbox {
     // Validate volumes: exactly one backend must be specified per volume
     if (opts.volumes) {
       for (const vol of opts.volumes) {
-        const backendsSpecified = [vol.host, vol.pvc, vol.ossfs].filter((b) => b !== undefined).length;
+        const backendsSpecified = [vol.host, vol.pvc, vol.ossfs].filter((b) => b != null).length;
         if (backendsSpecified === 0) {
           throw new Error(
             `Volume '${vol.name}' must specify exactly one backend (host, pvc, ossfs), but none was provided.`

sdks/sandbox/javascript/tests/sandbox.create.test.mjs

@@ -216,3 +216,26 @@ test("Sandbox.create rejects volume with multiple backends", async () => {
     /must specify exactly one backend \(host, pvc, ossfs\)/
   );
 });
+
+test("Sandbox.create treats null backends as absent", async () => {
+  const { adapterFactory, recordedRequests } = createAdapterFactory();
+
+  await Sandbox.create({
+    adapterFactory,
+    connectionConfig: { domain: "http://127.0.0.1:8080" },
+    image: "python:3.12",
+    skipHealthCheck: true,
+    volumes: [
+      {
+        name: "host-with-null-ossfs",
+        host: { path: "/tmp" },
+        ossfs: null,
+        pvc: undefined,
+        mountPath: "/mnt",
+      },
+    ],
+  });
+
+  assert.equal(recordedRequests.length, 1);
+  assert.equal(recordedRequests[0].volumes[0].host.path, "/tmp");
+});