Implement token balance checks in ContractClient

[Lakes41]

ContractClient.getMembershipTokenBalance is currently a stub that throws NOT_IMPLEMENTED. Implement token balance checking using an on-chain provider.

Suggested tasks

• Add a provider integration using viem or another agreed library
• Use the configured rpcUrl
• Validate wallet and contract addresses
• Query token balance from the contract
• Return the balance as a string
• Add tests with a mocked provider

Acceptance criteria

• getMembershipTokenBalance no longer throws NOT_IMPLEMENTED
• Token balances can be queried through the SDK
• Missing rpcUrl or contract address produces a clear error
• Provider logic is t

[leocagli]

I've analyzed the scope and I'm confident I can deliver. Please assign me!

[gloskull]

Hi, I’d love to work on this issue. I have 4 years of experience across backend, frontend, full-stack, and smart contract development. I understand the task clearly and can deliver a clean, efficient, well-tested solution on time, with strong focus on code quality and maintainability. My PR will be submitted in under 10 hours.

[grantfox-oss]

🦊 GrantFox — @leocagli has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #11)
2. Your PR will be reviewed by the Adamantine-guild maintainers

Good luck! Track your progress on GrantFox.

[leocagli]

Antes de abrir una PR revisé el código actual y este issue ya está implementado y mergeado (vía #66), así que prefiero confirmar el alcance antes de duplicar.

ContractClient.getMembershipTokenBalance (en src/contracts/contractClient.ts) ya no es un stub — no queda ningún NOT_IMPLEMENTED. Hoy:

• Valida walletAddress y contractAddress (validateAddress), y lanza GuildPassError(INVALID_CONFIG) claro si falta rpcUrl o contractAddress.
• Consulta on-chain vía eth_call con el selector balanceOf(address) (${BALANCE_OF_SELECTOR}${encodeAddressArgument(walletAddress)}) usando el rpcUrl configurado y el HttpClient (sin sumar una dependencia pesada como viem — usa JSON-RPC crudo + js-sha3).
• Decodifica el uint256 y lo devuelve como string (decodeUint256Result), y propaga errores del provider como GuildPassError(HTTP_ERROR).
• Está cubierto por ~15 casos en tests/contracts.test.ts (provider mockeado).

Eso cubre toda la AC (no NOT_IMPLEMENTED, balances consultables, errores claros de rpcUrl/contract, lógica testeada). ¿Querés que cierre este issue como duplicado de #66, o hay una extensión concreta que tengas en mente? Posibles gaps reales sobre los que con gusto mando PR: un getMembershipTokenBalances batch (apoyándose en los helpers de eth_call batch de #107), o exponer metadata ERC-20 (decimals/symbol) para formatear el balance. Avisame y lo implemento.