test(e2e): retry dataset downloads in onPrepare

Flaky connectivity to data.kitware.com could fail a single onPrepare
download, leaving .tmp without the dataset and causing specs to fail at
the render-wait step as if WebGL were broken. Retry each download up to
3 times, write atomically via a .part rename so a partial download is
never cached as complete, and check response.ok so an error page is not
saved as DICOM.

Author: PaulHax

wdio.shared.conf.ts

@@ -111,14 +111,45 @@ export const config: Options.Testrunner = {
   async onPrepare() {
     fs.mkdirSync(TEMP_DIR, { recursive: true });
 
+    const RETRIES = 3;
+    const RETRY_DELAY_MS = 500;
+    const delay = (ms: number) =>
+      new Promise((resolve) => {
+        setTimeout(resolve, ms);
+      });
+    const downloadOnce = async (url: string, savePath: string) => {
+      const response = await fetch(url);
+      if (!response.ok) {
+        throw new Error(`HTTP ${response.status} for ${url}`);
+      }
+      const data = await response.arrayBuffer();
+      // Write to a temp path first so a failed/partial download never leaves a
+      // corrupt file that the existsSync check would treat as already cached.
+      const tmpPath = `${savePath}.part`;
+      fs.writeFileSync(tmpPath, Buffer.from(data));
+      fs.renameSync(tmpPath, savePath);
+    };
+
     const downloads = TEST_DATASETS.map(async ({ url, name }) => {
       const savePath = path.join(TEMP_DIR, name);
       if (fs.existsSync(savePath)) {
         return;
       }
-      const response = await fetch(url);
-      const data = await response.arrayBuffer();
-      fs.writeFileSync(savePath, Buffer.from(data));
+      for (let attempt = 1; attempt <= RETRIES; attempt += 1) {
+        try {
+          await downloadOnce(url, savePath);
+          return;
+        } catch (err) {
+          if (attempt === RETRIES) {
+            throw new Error(
+              `Failed to download ${name} after ${RETRIES} attempts: ${
+                (err as Error).message
+              }`
+            );
+          }
+          await delay(RETRY_DELAY_MS);
+        }
+      }
     });
     await Promise.all(downloads);
   },