Add Savings Vault negative-path tests

[El-swaggerito]

Description

Difficulty: Expert

Type: Test

Summary

Add negative-path tests for the Savings Vault contract.

Current Behaviour

Vault tests may focus on successful deposit and withdrawal flows while missing invalid or unauthorised cases.

Expected Behaviour

The test suite should cover invalid deposit amounts, early withdrawals, unauthorised access, missing initialisation, and invalid lock states.

Suggested Implementation

Create negative-path tests, assert expected errors, and verify state remains unchanged after failure.

Files or Areas Likely Affected

• contracts/
• src/
• tests/
• docs/

Acceptance Criteria

• Invalid deposit amount tests are added.
• Early withdrawal tests are added.
• Unauthorised access tests are added.
• Missing initialisation tests are added where applicable.
• State consistency after failures is verified.
• Docs reference negative-path coverage.

Additional Notes

This makes contract PRs more evaluation-ready.

[abbarrh01]

I have experience in React and I have reviewed the code. I will submit a PR as soon as I am assigned

[grantfox-oss]

🦊 GrantFox — @abbarrh01 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #489)
2. Your PR will be reviewed by the Axionvera maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@abbarrh01's PR #521 was approved and merged by @El-swaggerito.

🏆 @abbarrh01: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (223 total points). Track your full progress on GrantFox.

👏 Great work, @abbarrh01! Keep contributing to Axionvera.