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/mediaManager.integration.test.ts

@@ -72,6 +72,7 @@ describe("MediaService V2 Integration Logic", () => {
              moveTo: jest.fn(),
              getMimeType: () => "image/jpeg",
              getBlob: () => ({}),
+             getSize: () => 1024,
              getId: () => id
         }))