feat: add extension update check

tests/background-index.test.ts

@@ -48,6 +48,50 @@ describe("background-index", () => {
     expect(sendResponse).toHaveBeenCalledWith({ ok: true });
   });
 
+  test("downloads extension update assets", async () => {
+    const listeners: Array<
+      (message: unknown, sender: unknown, sendResponse: (response: unknown) => void) => boolean | void
+    > = [];
+    const download = vi.fn(async () => undefined);
+    const sendResponse = vi.fn();
+
+    registerBackgroundMessageHandler({
+      downloads: {
+        download
+      },
+      runtime: {
+        onMessage: {
+          addListener(listener) {
+            listeners.push(listener);
+          }
+        }
+      }
+    });
+
+    const result = listeners[0](
+      {
+        filename: "star-chart-search-enhancer-internal.zip",
+        type: "update:download",
+        url: "https://cos.example.com/star-chart-search-enhancer/releases/0.2.0421.3/star-chart-search-enhancer-internal.zip"
+      },
+      {},
+      sendResponse
+    );
+
+    expect(result).toBe(true);
+    await new Promise((resolve) => setTimeout(resolve, 0));
+
+    expect(download).toHaveBeenCalledWith({
+      filename: "star-chart-search-enhancer-internal.zip",
+      saveAs: true,
+      url: "https://cos.example.com/star-chart-search-enhancer/releases/0.2.0421.3/star-chart-search-enhancer-internal.zip"
+    });
+    expect(sendResponse).toHaveBeenCalledWith({
+      ok: true,
+      type: "update:download-ack"
+    });
+  });
+
   test("responds to auth:get-state with auth status", async () => {
     const listeners: Array<
       (message: unknown, sender: unknown, sendResponse: (response: unknown) => void) => boolean | void