[BUG]: Negative transaction fees allow arbitrary balance inflation

[Jiyacodex]

Bug Description

State.verify_transaction_logic() and State.validate_and_apply() do not validate that tx.fee is non-negative before calculating the transaction cost.
As a result, a transaction with a negative fee (e.g. fee = -1000, amount = 0) is accepted, causing total_cost to become negative. When the transaction is applied, the sender's balance increases instead of decreasing because a negative value is subtracted from the balance.

I reproduced this locally on the current codebase: a transaction with amount = 0 and fee = -1000 was accepted successfully, increasing the sender's balance from 10 to 1010. This allows arbitrary balance inflation through invalid transactions.

Steps to Reproduce

1. Create an account with a positive balance (e.g., 10 coins).

2. Construct and sign a transaction with:
   amount = 0
   fee = -1000
   a valid nonce
   a valid chain_id

3. Submit the transaction through State.validate_and_apply().

4. Observe that the transaction succeeds and the sender's balance increases instead of remaining unchanged.

Logs and Screenshots

Environment Details

OS: macOS
Python Version: 3.12.13
Virtual Environment: Python venv (.venv)
Project: MiniChain (latest main branch)
Execution Method: Local reproduction using a minimal Python PoC invoking State.validate_and_apply()

Impact

High - Major feature is broken

Code of Conduct

• I have joined the Discord server and will post updates there
• I have searched existing issues to avoid duplicates