Fix Drive video upload to Shopify

- Detect video mime types in MediaService to set correct resource ('VIDEO') and mediaContentType.

- Add fileSize to stagedUploadsCreate payload as required by Shopify for videos.

- Add safety check for missing upload targets to prevent crashes.

- Implement getSize in MockDriveService.

- Add unit test in MediaService.test.ts to verify correct resource and fileSize handling for video uploads.

- Update mock in mediaManager.integration.test.ts to support getSize().

src/services/MockDriveService.ts

@@ -52,6 +52,7 @@ export class MockDriveService implements IDriveService {
         getThumbnail: () => ({ getBytes: () => [] }),
         getMimeType: () => (blob as any).getContentType ? (blob as any).getContentType() : "image/jpeg",
         getDownloadUrl: () => `https://drive.google.com/uc?export=download&id=${id}`,
+        getSize: () => blob.getBytes ? blob.getBytes().length : 0,
         getAppProperty: (key) => {
              return (newFile as any)._properties?.[key]
         }