test: cover vault withdrawal flow

[pntech20]

Summary

• add focused coverage for the vault withdrawal flow, including success, wallet wiring, invalid amounts, submission errors, and simulation errors
• normalize deposit and withdrawal amounts before building Soroban calls so invalid values fail before transaction submission
• fix the cache test mock typing and account setup so the full Jest suite runs cleanly

Checks

• npm test -- --runInBand
• npm run build
• git diff --check

Closes #23