Implement Savings Vault withdrawal guard tests

[El-swaggerito]

Description

Difficulty: Expert

Type: Test

Summary

Add comprehensive withdrawal guard tests for the Savings Vault contract.

Current Behaviour

Withdrawal behaviour may not be fully tested for locked funds, early withdrawals, unauthorised callers, and insufficient balances.

Expected Behaviour

The contract test suite should verify all major withdrawal restrictions and state consistency after failure.

Suggested Implementation

Add negative-path tests, success-path tests, state assertions, and documentation notes.

Files or Areas Likely Affected

• contracts/
• src/
• tests/
• docs/

Acceptance Criteria

• Early withdrawal tests are added.
• Unauthorised withdrawal tests are added.
• Insufficient balance tests are added.
• State remains unchanged after failed withdrawals.
• Successful withdrawal behaviour is still tested.
• Docs reference withdrawal guard coverage.

Additional Notes

This is a meaningful contract testing issue.