SC-12: Add reentrancy guards to `payout_tournament` and all fund transferring functions

[chinweobtagaz]

Context: Although Soroban is generally safer against reentrancy, defending against it explicitly is a best practice for complex state updates.
Location: contracts/game_contract/src/lib.rs
Acceptance Criteria:

• Implement a nonReentrant modifier/check mechanism during token payouts.

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