Add Savings Vault deposit boundary tests

[El-swaggerito]

Description

Difficulty: Expert

Type: Test

Summary

Add boundary tests for Savings Vault deposit behaviour.

Current Behaviour

Deposit logic may not be tested for zero values, very large values, repeated deposits, and invalid initialisation states.

Expected Behaviour

The test suite should verify deposit boundaries and ensure state remains consistent.

Suggested Implementation

Create boundary tests, invalid input tests, repeated deposit tests, and documentation updates.

Files or Areas Likely Affected

• contracts/
• src/
• tests/
• docs/

Acceptance Criteria

• Zero deposit tests are added.
• Large value deposit tests are added where practical.
• Repeated deposit behaviour is tested.
• Invalid initialisation state is tested.
• State consistency is verified.
• Docs explain deposit assumptions.

Additional Notes

This improves contract confidence.