fix: resolve the flight data before the RSC cancel in the react-router patch

[Mheaus]

#6 was merged before this commit reached its branch. It aligns patches/react-router@8.4.0.patch with the latest commit of remix-run/react-router#15612.

In production, the RSC stream is a tee() branch, and its cancel waits for the other branch. stop() now unblocks flush() before it cancels the RSC stream. Before this change, an abort during flush() kept the response open until the RSC render ended.

Verification on the demo production build, with requests that the client aborts after 1–30 ms:

• 300 requests, one at a time: 0 stops;
• 1,000 requests, 20 in parallel: 0 stops, no error in the log.