feat: implemented the SEP-10 challenge verification feature

[CollinsC1O]

Summary

• Added Storage Keys: Created AUTH_NONCE to handle replay protections against nonces being used multiple times and a NODE_REG map to accurately track node authorization sequences and timestamp states on-chain.

• Implemented Verification Logic: Wrote the core verify_sep10_challenge function in lib.rs. The code builds a secure SHA-256 payload composed of the b"SEP10" prefix, node public key, challenge nonce, and timestamp prior to checking it against the node's provided signature via env.crypto().ed25519_verify(...).

• Time Bounds Safety Check: Factored in bounds protection to throw an explicit ContractError::AuthTimeout if the timestamp on the challenge payload expires or passes an unacceptably delayed time window (timestamp +/- 300 seconds using env.ledger().timestamp()). This avoids permanently stalled, stale authorizations while restricting CPU load.

• Analytics Support: Exposed an get_node_auth helper mapped with a corresponding standard env.events().publish((symbol_short!("node_auth"), node_pubkey), timestamp); event emitter to keep the off-chain/backend services optimally synced.

Closes #519

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• contracts/game_contract/src/lib.rs

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