Mock Stellar RPC for VaultModule unit tests

[Code-Paragon]

Problem

The VaultModule is currently untested. To prevent tests from attempting real testnet network requests, the Stellar RPC must be mocked to test deposit and withdraw logic safely.

Expected behavior

A new test file verifies that VaultModule.deposit properly formats stroop conversion, builds the transaction, and returns a mock transaction hash using Jest mocks on rpc.Server.

Files to update

• tests/vault.test.ts (update/expand)

Project relevance

Allows developers to confidently refactor the core financial logic of the SDK without worrying about network latency or live testnet token balances during testing.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• client.rpcServer.sendTransaction is successfully mocked to return a static { hash: "mock_hash" } payload
• No TypeScript, lint, test, or build errors