Fix the money transfer between two account balances

You asked an agent for thorough tests of a bank transfer, and it wrote six copies of one happy path: move 100, check both balances. The team now wants the transfer rules in one function, so every edge case can be tested directly.

An assistant wrote apply_transfer(balances, source_index, target_index, amount). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• balances: each account's balance in cents, as integers. The list can be empty.
• source_index: the position of the account that pays. It can be negative or past the end.
• target_index: the position of the account that receives. It can be negative or past the end.
• amount: the amount to move, in cents, an integer. It can be 0 or negative.
• Return a new list with every balance after the transfer.

Rules the tests check:

• A transfer is valid only when all of these hold:
  • amount is greater than 0.
  • Both indices are between 0 and the length of balances minus 1.
  • The two indices are different.
  • The source balance is at least amount. Moving the whole balance is allowed.
• A valid transfer subtracts amount from the source and adds it to the target. Every other balance stays the same.
• An invalid transfer changes nothing. Return the balances exactly as they were.
• An empty balances returns an empty list.