test(contracts): add unit tests for InvoiceTokenContractService.buildMintTx()

[TS-mfon]

closes #155

Summary of Changes

This PR implements the InvoiceTokenContractService for building Soroban SEP-41 mint transactions and adds comprehensive unit test coverage in tests/unit/services/stellar/invoice-token-tx.test.ts.

Changes Included

• InvoiceTokenContractService (src/services/stellar/invoice-token-contract.service.ts):
  • Implements buildMintTx(recipientAddress, tokenAmount) to construct SEP-41 mint contract calls.
  • Converts recipient addresses to Address.toScVal() and token amounts to nativeToScVal(amount, { type: 'i128' }).
• Unit Test Suite (tests/unit/services/stellar/invoice-token-tx.test.ts):
  • Verifies generated operation type is invokeHostFunction.
  • Verifies contract address matches configured TOKEN_CONTRACT_ID.
  • Asserts ScVal serialization of recipient address and token supply (i128).
  • Tests support for multiple numeric input formats (bigint, number, string).
  • Tests boundary conditions (zero amounts, large 128-bit values) and invalid address rejection.

Verification

• Added test coverage in tests/unit/services/stellar/invoice-token-tx.test.ts.
• Validated TypeScript typing and structure across all added modules.

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