test: add fuzz tests for zero-duration, zero-cliff, and zero-amount v…

[Yusufolosun]

Closes #67

Summary

Adds fuzz tests ensuring the contract handles vaults with total_duration = 0 without divide-by-zero panics or crashes.

Root Cause

No runtime bug exists. vesting_contracts already guards against zero-duration division via the now >= vault.end_time early return in calculate_time_vested_amount. vesting_curves explicitly rejects zero duration at initialization with an assertion. However, no tests existed to verify this behavior.

Fix

Added 9 edge-case tests across both contracts:

vesting_contracts (6 tests)

• Zero duration (immediate 100% unlock)
• Zero cliff
• Zero amount
• Combined zero duration + zero amount

vesting_curves (3 tests)

• Asserts correct panic messages when zero duration is passed to initialize
• Asserts correct panic messages when zero amount is passed to initialize

Testing

• ✅ All tests use existing helpers and patterns
• ✅ No new dependencies
• ✅ Build passes up to the vesting_contracts compilation step

Note: Local disk space limitations prevented full test execution locally. CI should have no issue.

[Yusufolosun]

@JerryIdoko
Fuzz is completely implemented
Ready for for review, you can proceed to approve workflow