fix(upload): fail fast on 0-byte files in multipartUpload

A 0-byte file produced a multipart upload with zero parts, which S3/R2 reject
with a cryptic CompleteMultipartUpload error. Guard file.size === 0 up front
with a clear message, before any S3 request is issued. Cloudflare-upload #4.

Test: tests/multipart-empty-file-guard.test.mjs.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

js/storage/s3-multipart.js

@@ -177,6 +177,11 @@ export async function multipartUpload({
   if (!file) {
     throw new Error("File is required");
   }
+  // A 0-byte file produces a multipart upload with zero parts, which S3/R2
+  // rejects with a cryptic CompleteMultipartUpload error. Fail fast and clearly.
+  if (typeof file.size === "number" && file.size === 0) {
+    throw new Error("File is empty (0 bytes). Choose a non-empty file to upload.");
+  }
 
   const {
     CreateMultipartUploadCommand,

tests/multipart-empty-file-guard.test.mjs

@@ -0,0 +1,38 @@
+// Scenario (SCN-upload-empty-file):
+//   Given a 0-byte file,
+//   When multipartUpload is called,
+//   Then it rejects FAST with a clear "empty" message and never issues any S3
+//     request (the old behavior completed a multipart upload with zero parts and
+//     surfaced a cryptic CompleteMultipartUpload error — Cloudflare-upload #4).
+
+// Disable the network import of the AWS SDK before importing the module.
+globalThis.__BITVID_DISABLE_NETWORK_IMPORTS__ = true;
+
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { multipartUpload } = await import("../js/storage/s3-multipart.js");
+
+test("rejects a 0-byte file before any S3 request", async () => {
+  let sendCalled = false;
+  const s3 = {
+    send: async () => {
+      sendCalled = true;
+      return {};
+    },
+  };
+
+  await assert.rejects(
+    () =>
+      multipartUpload({
+        s3,
+        bucket: "bucket",
+        key: "u/npub/ns/empty.mp4",
+        file: { size: 0, type: "video/mp4" },
+      }),
+    /empty|0 bytes/i,
+    "empty file should produce a clear error",
+  );
+
+  assert.equal(sendCalled, false, "must not start a multipart upload for an empty file");
+});