feat: implement pool cancellation feature with admin controls and related events

[No-bodyq]

Implements pool cancellation functionality allowing admins to cancel active prediction pools, freezing all betting activity and preventing new predictions from being placed.

Changes:

• Added canceled: bool field to Pool struct
• Implemented cancel_pool() function with admin-only authorization
• Added PoolCanceled error type and PoolCanceledEvent
• Prevents placing predictions on canceled pools
• Prevents canceling already-resolved pools
• All 26 contract tests passing

closes #309

[vercel]

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[No-bodyq]

Kindly review @Yunusabdul38