Add zero-value edge case tests for deposits and withdrawals

[Code-Paragon]

Problem

The CONTRIBUTING.md strictly mandates testing for zero-value edge cases, but test.rs currently only tests standard positive values (e.g., 1000 and 500).

Expected behavior

New test cases are added to explicitly pass 0 and negative integers into the deposit and withdraw methods to verify the amount > 0 assertions trigger correctly.

Files to update

• src/test.rs

Project relevance

This satisfies the strict testing requirements outlined for contributors and prevents potential accounting manipulation vectors in the Vault.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• Explicit test cases verify that deposit(&user, &0) panics with the strictly positive error message
• No TypeScript, lint, test, or build errors