Add DisputeModule.expireDispute() to force-expire expired disputes

[merlik787-droi]

Summary

Implemented expireDispute() - admin-only method to force-expire an open dispute.

Changes

• Added expireDispute(disputeId) to DisputeModule - Requires signing keypair - Validates dispute exists and is still open - Calls contract expire_dispute method - Added 3 unit tests

Closes #234

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[BigBen-7]

This PR currently has merge conflicts with main and was skipped during the batch merge pass. Please rebase/resolve conflicts and it can be merged.