test: add unit tests for ERC-20 invoice coverage

[eshaanag]

Description

This PR introduces comprehensive unit tests for the ERC-20 invoice creation, payment, and batch payment code paths in the Chainvoice contract.

Previously, only native ether invoice workflows were tested in the suite. This change ensures that the contract's token interactions (balance verification, allowance checking, and transfers) are fully covered and verified.

Changes Made

• Introduced a MockERC20 token helper inside the test file contracts/test/Chainvoice.t.sol.
• Added testCreateInvoice_ERC20(): Verifies successful ERC-20 invoice creation.
• Added testPayInvoice_ERC20(): Verifies successful invoice payment via approved ERC-20 tokens.
• Added testPayInvoice_ERC20_RevertInsufficientAllowance(): Asserts that payments revert when the payer has not approved the contract.
• Added testPayInvoice_ERC20_RevertFeeMustBeNative(): Asserts that payments revert if the native fee is not sent.
• Added testPayInvoicesBatch_ERC20(): Verifies batch payment processing for multiple ERC-20 invoices.

Verification

All 33 contract tests pass successfully under Foundry:

forge test

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