feat(sdk): sync createPayment ABI, export fee types, add merchant/refund/dispute methods

[miss-yusrah]

PR description

Summary

Brings the TypeScript SDK in line with the current FluxaPay contract ABI and adds the missing high-level client methods for merchant management, refunds, and disputes.

Changes

Task 1 — Fix createPayment signature

• Added CreatePaymentArgs, FeeConfig, and MaybeFeeConfig to contract bindings in sdk/src/contracts/fluxapay/src/index.ts
• Updated FluxapayClient.createPayment() to accept optional durationSecs, memo, memoType, tokenAddress, and clientToken
• expiresAt is now optional (matches contract Option<u64>)
• Contract calls pass a full CreatePaymentArgs object via toCreatePaymentArgs()
• Exported CreatePaymentArgs, FeeConfig, and MaybeFeeConfig from the SDK index

Task 2 — Export fee types and registerMerchant

• FeeConfig and MaybeFeeConfig are exported from @fluxapay/sdk
• FluxapayClient.registerMerchant() accepts an optional feeConfig: FeeConfig
• README includes register-with-fee and register-without-fee examples

Task 3 — Merchant management methods

• Added merchantRegistryContractId to FluxapayConfig for a dedicated MerchantRegistry contract
• FluxapayClient methods:
  • registerMerchant(params) — matches MerchantRegistry.register_merchant
  • updateMerchant(params) — business name, currency, active, payout, bank, fee config
  • getMerchant(merchantId)
  • verifyMerchant(admin, merchantId)
• Methods delegate to MerchantRegistryClient when merchantRegistryContractId is set; otherwise use the main contract
• MerchantRegistryClient updated with typed RegisterMerchantParams / UpdateMerchantParams and full fee config support

Task 4 — Refund and dispute methods

Added to FluxapayClient:

Method | Contract call -- | -- processRefund(operator, refundId) | process_refund getRefund(refundId) | get_refund getPaymentRefunds(paymentId) | get_payment_refunds createDispute(...) | create_dispute reviewDispute(operator, disputeId) | review_dispute resolveDisputeWithRefund(operator, disputeId, notes) | resolve_dispute_with_refund rejectDispute(operator, disputeId, notes) | reject_dispute getDispute(disputeId) | get_dispute getPaymentDisputes(paymentId) | get_payment_disputes

closes #382
closes #383
closes #385
closes #386

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