test: add can_withdraw default behaviour tests

[ghg001]

Overview

This PR adds explicit tests for can_withdraw default behaviour in the savings vault. It verifies the function returns predictable values for a new user with no balance, a user with an available balance but no lock, and a user after a lock has been set up where applicable. The default logic in contracts/savings_vault/src/lib.rs is also made explicit so the tested defaults are intentional rather than incidental.

Related Issue

Closes the bounty issue: Add tests for can_withdraw default behaviour

Changes

🧪 Explicit can_withdraw Default Tests

• [ADD] contracts/savings_vault/src/test/mod.rs

  • Registers the new default-behaviour test modules.

• [ADD] contracts/savings_vault/src/test/withdraw_lock.rs

  • Test: can_withdraw_new_user_returns_false — no balance, no lock, expected default false.
  • Test: can_withdraw_available_balance_no_lock_returns_true — funds available, no lock, expected default true.

• [ADD] contracts/savings_vault/src/test/lock_maturity_boundary.rs

  • Test: can_withdraw_locked_balance_before_unlock_returns_false — lock created but not yet mature, expected false.
  • Test: can_withdraw_after_unlock_returns_true — boundary check for comparison.

• [MODIFY] contracts/savings_vault/src/lib.rs

  • Makes default can_withdraw handling explicit for zero-balance and no-lock cases.
  • Keeps existing lock/unlock behaviour unchanged.

• [MODIFY] contracts/savings_vault/src/test/test_helpers.rs

  • Adds deterministic helper setup for users with and without balances/locks.

Verification Results

cargo test -p savings_vault can_withdraw
✅ 4/4 passed

Manual expectations:
✅ New user no balance → can_withdraw = false
✅ Available balance, no lock → can_withdraw = true
✅ Locked balance before unlock → can_withdraw = false
✅ Deterministic setup with fixed timestamps

Acceptance Criteria	Status
can_withdraw for a new user is tested	✅ can_withdraw_new_user_returns_false
can_withdraw for a user with available balance and no lock is tested	✅ can_withdraw_available_balance_no_lock_returns_true
Expected default result is asserted	✅ Asserted on every new default-state test
Tests use deterministic setup	✅ Helper-created balances and blocks with fixed timestamps
Test names describe each state clearly	✅ Names encode user state and expected result

Closes #100