test: add unit tests for vault interaction methods with mocked StellarSDK dependencies

[shamsss04]

Soroban Vault Boundary Tests

Closes #188

Summary

This PR introduces comprehensive boundary tests for the Soroban Savings Vault module (src/soroban/index.ts). These tests validate that the SDK interacts correctly with the deployed smart contract, handles inputs/outputs precisely, and wraps any simulated or network errors robustly.

Motivation

To ensure the reliability of the PocketPay SDK when interacting with the Soroban smart contracts, we need to guarantee that the depositToVault, withdrawFromVault, and getVaultBalance functions work seamlessly under various network constraints and handle boundaries appropriately.

What's New

New Test Suite (tests/vault.test.ts)

A dedicated unit testing suite was created for the Soroban methods, utilizing robust vi.mock techniques to simulate Horizon/Soroban RPC behavior without any live network dependencies. The mocked operations explicitly document the expected real-network state transformations.

Method	Boundaries Tested
depositToVault	Contract interaction (deposit), invalid secret validation, success mappings, simulation errors, unhandled RPC wrapper handling.
withdrawFromVault	Contract interaction (withdraw), amount validation, success mappings, and transaction submission error handling.
getVaultBalance	Contract interaction (get_balance), public key validation, timeout/offline wrapping, and parsing scValToNative back to an XLM string.

Files Changed

File	Change
tests/vault.test.ts	New — 13 tests covering Vault boundary states, input/output validation, and error mappings.
src/soroban/index.ts	(No changes required for tests to pass, existing logic was verified)

Acceptance Criteria

• Vault boundary tests are added.
• Expected method names are covered.
• Input and output shapes are tested.
• Contract error mapping is tested.
• Mocked versus real behaviour is documented inside the tests.

Verification

npm run test          # Full test suite - 13/13 passing in tests/vault.test.ts