fix: validate payment_open_at is before payment_close_at (#697)

[deborahairtel887-dotcom]

Summary

Validates that payment_open_at < payment_close_at when both fields are configured on InvoiceOptions2 during create_invoice. If payment_open_at >= payment_close_at, create_invoice now returns ContractError::InvalidAmount.

Changes

• In contracts/split/src/lib.rs, validate that payment_open_at < payment_close_at when both are Some(_), returning ContractError::InvalidAmount if payment_open_at >= payment_close_at.
• Ensures no error is raised when only one of the fields is set or when both are None.
• Added unit tests in contracts/split/src/test.rs:
  • test_create_invoice_payment_open_at_greater_than_close_at_rejected: verifies rejection with ContractError::InvalidAmount for open_at = 100, close_at = 99.
  • test_create_invoice_payment_open_at_equal_to_close_at_rejected: verifies rejection with ContractError::InvalidAmount for open_at = 100, close_at = 100.
  • test_create_invoice_payment_open_at_less_than_close_at_ok: verifies acceptance for open_at = 99, close_at = 100.
  • test_create_invoice_payment_window_only_one_or_none_ok: verifies invoice creation succeeds when only one field is set or both are None.

closes #697

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