feat(contract): add cancellation window constraints

[lycantho]

[SC-020] Add Cancellation Window Constraints

Description

This PR addresses issue #20 by implementing cancellation window constraints for time slot tokens in the ChronoPay smart contract, ensuring buyers are prevented from cancelling a time slot too close to the start time.

Core Changes

• New storage keys: CancelWindowSecs, SlotBuyer(u32), and SlotStartTime(u32) to keep track of slot configuration, buyer records, and scheduled start times.
• buy_time_slot: A robust handler complementing the old stub, registering the buyer for a slot.
• set_cancel_window: Allows the contract owner to configure the allowed cancellation window prior to start time (defaulting to 3600 seconds/1 hour).
• cancel_time_slot: Enables a buyer to cancel their reservation if requested before the designated cancellation window starts. If called too late, or for non-existent/unsold slots, the transaction panics. Safely resets the slot status to Available upon success.

Testing

• Wrote 8 exhaustive unit tests rigorously evaluating expected behaviors and explicit panic! conditions (e.g., zero windows, cancelling post-deadline, cancelling unassigned slots).
• Achieved ~100% test coverage for all new conditions and failure modes under touched modules.

Security and Architecture Notes

• Storage manipulation efficiently restricts status modifications and handles default fallback states securely without overriding bounds logic.

Closes #20.

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[lycantho]

@thlpkee20-wq please merge

[lycantho]

@thlpkee20-wq

[lycantho]

@thlpkee20-wq