fixture: add contract mocks for Hardhat test suites

[devmocrea]

Description

Create clean contract mocks in apps/contracts/test/mocks. Build mock implementations of standard ERC20 tokens to verify payment integration tests.

Technical Specification

• Add a contract MockERC20.sol in apps/contracts/contracts/mocks.
• Inherit OpenZeppelin's ERC20 implementation.
• Expose a public mint function for testing purposes.

Acceptance Criteria

• Tests can mint and approve mock tokens to thoroughly test Celo ERC20 flows.