feat: add match cancellation fee to discourage abuse

[AbuJulaybeeb]

What was done

• Added cancellation_fee_basis_points: u32 to the ProtocolConfig structure with a default value of 100 (1%).
• Updated the cancel_match logic to calculate the cancellation fee using the formula: stake_amount * cancellation_fee_basis_points / 10_000.
• Implemented the deduction of the calculated fee from the cancelling player's deposit prior to refunding the remaining balance.

Why it was done

Previously, players could cancel pending matches for free, which created a DoS vulnerability through spam cancellations. Introducing a small cancellation fee economically disincentivizes this abusive behavior and ensures protocol stability.

How it was verified

• Verified fee calculation math correctly applies the configured basis points to the player's stake.
• Confirmed the fee is correctly deducted from the refund amount.
• Validated that the remaining balance is successfully refunded to the cancelling player.
• Executed the test suite (cargo test) to ensure all cancellation flow tests and invariants pass successfully.

Closes #951

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