Check the balance of an account

[huangweili97]

As an existing bank customer with an account, I want to check the balance of my account, so that I can know how much money is available.

Acceptance Criteria

Scenario 1: Successful balance check
Given the user has an active bank account,
When they provide the correct account ID,
Then the system returns the current balance of the account.

Scenario 2: Unsuccessful balance check due to invalid account ID
Given the user provides an invalid or non-existent account ID,
When they attempt to check the balance,
Then the system does not provide the balance and an error message is displayed.

Tasks

1. Add a method for checking the balance of an account in the Bank class.
2. Ensure the method validates the account ID before returning the balance.
3. Create unit tests to verify all acceptance criteria.