feat: implement multi-token support with token management functions and associated events

[bytebinders]

What was done:

• Event Addition: Defined InvoiceTokenChanged in events.rs to provide an on-chain audit trail for currency switches.
• Core Implementation: Added convert_invoice_token() to the main contract in lib.rs.
  • Permissions: Restricted to the original invoice submitter (freelancer).
  • State Guard: Only allowed while the invoice is in the Pending state.
  • Allowlist Check: Ensures the new token is approved by governance.
  • Expiry Check: Automatically marks the invoice as Expired if the conversion is attempted after the due date.
• Unit Testing: Created tests_token_switch.rs with the following test cases:
  • Success: Switching from USDC to EURC.
  • Failure: Switching by a non-submitter.
  • Failure: Switching to a non-allowlisted token.
  • Failure: Switching after the invoice has been funded (even partially).
  • Failure: Switching after the invoice has expired.

Why it was done:

To improve flexibility for both freelancers and payers. This allows a freelancer to adapt to a payer's currency preference (e.g., switching to EURC if the payer holds EURC) as long as the invoice has not yet been funded by a liquidity provider.

How it was verified:

• Implemented comprehensive unit tests covering success paths and all state/permission guards.
• Verified that the logic mirrors the security patterns used in update_invoice and fund_invoice.

---

Summary of Changes:

File	Action	Description
events.rs	Modified	Added InvoiceTokenChanged event struct.
lib.rs	Modified	Implemented convert_invoice_token and registered test module.
tests_token_switch.rs	Created	Added comprehensive unit tests for the new feature.

Closes #21

[drips-wave]

@bytebinders Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits