Griefing Attack: claim unfilled request to block withdraw after expiry

If a request is not filled, anyone can claim it and stop it from being withdrawn by the user until the challenge period is over.
Takes at least, finalization time of target roll up.

Bad user experience.

No files, tests, or entry points are named. Trace the claim, expiry, challenge-period, and withdrawal flows to confirm the reported griefing path, then define the intended behavior and tests that demonstrate an unfilled request can still be withdrawn after expiry.

Written by the indexing model from the issue text.