Fix Hinkal Private Payment Unit Tests to not leak private key and run nightly

[MantisClone]

Problem

The Hinkal Private Payments integration tests call external APIs and use real funds on Base. Furthermore, they leak the private key.

Proposed solution

• Fix the integration tests to use environment variables
• Rotate the private key and store as Github Action Secret
• Top up the new address with USDC and ETH on Base
• Configure the Hinkal Private payment tests to run nightly monthly

[MantisClone]

New Payer address: https://basescan.org/address/0xAb93553Aa6A889A5F0a890F6123fbda7890D7417
New Payee address: https://basescan.org/address/0xb6A17Fd9681fb036E09b3C251fa7bA6bA045eA7d