feat: Implement deposit_fiat and withdraw_fiat Functions for Fiat Tr…

[Olowodarey]

Add Fiat Deposit and Withdrawal Functionality

Description

This PR implements fiat currency deposit and withdrawal functionality in the Account contract, including comprehensive test coverage.

Changes

• Added deposit_fiat and withdraw_fiat functions to handle fiat currency operations
• Added storage for tracking fiat balances using a mapping of (user, currency) → amount
• Implemented event emission for both deposit and withdrawal operations
• Added input validation for zero amounts, zero addresses, and sufficient balances

Test Coverage

• test_deposit_fiat: Verifies basic deposit functionality
• test_withdraw_fiat_insufficient_balance: Ensures withdrawals can't exceed balance
• test_multiple_deposits: Tests multiple deposits to the same currency
• test_different_currencies: Verifies isolation between different currency balances
• test_deposit_zero_amount: Validates zero-amount deposit prevention

Security Considerations

• All operations are protected with assert_only_self()
• Input validation for zero values and addresses
• Balance checks before withdrawals
• Events emitted for all state changes

Notes

• Uses u128 for amounts to support large values
• Currency is represented as felt252 for flexibility
• Includes proper error messages for all failure cases

closes #3