Add expires_at field to invoices and reject payments on expired invoices

[AlonsoFi]

Closes: #114

Description

Add invoice expiry support via an optional expires_at: Option field on
invoices. When set, pay_invoice rejects payments after the expiry timestamp
with a new InvoiceExpired error. When None, invoices remain payable
indefinitely (backward-compatible).

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• Documentation update
• Code refactoring
• Test improvements
• Other (please describe):

Related Issues

Fixes

Changes Made

• Added expires_at: Option field to the Invoice struct in types.rs
• Added InvoiceExpired = 21 variant to ContractError in errors.rs
• Updated create_invoice and create_invoice_signed signatures in interface.rs,
  shade.rs, and components/invoice.rs to accept the new parameter
• Added expiry validation in pay_invoice: rejects payment if
  env.ledger().timestamp() > expires_at
• Updated signature_util.rs to include expires_at in the signed message
  (build_message and verify_invoice_signature)
• Updated all existing test call sites (~66 calls) across 7 test files to pass
  &None as expires_at
• Updated build_test_message and sign_invoice test helpers to include
  expires_at in signature construction
• Created test_invoice_expiry.rs with 3 new tests and registered it in
  tests/mod.rs

Testing

• I have added tests that prove my fix is effective or that my feature works
• All existing tests pass locally
• I have tested this manually

New tests in test_invoice_expiry.rs:

• test_pay_expired_invoice_fails — creates invoice with expiry, advances
  ledger past it, asserts payment panics with InvoiceExpired (Create Comprehensive Tests for Merchant Public Key Management #21)
• test_pay_invoice_before_expiry_succeeds — creates invoice with expiry, pays
  before it, asserts success
• test_invoice_no_expiry_always_payable — creates invoice with None expiry,
  pays at a far-future timestamp, asserts success

Result: 150 tests pass (118 shade + 32 account), 0 failures.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Additional Notes

The expires_at parameter is fully optional. Passing None preserves the
previous behavior — invoices without an expiry are payable at any time. The
expiry is also included in the signed message for create_invoice_signed,
ensuring it cannot be tampered with.

[codebestia]

@AlonsoFi Fix the Ci and resolve the conflicts.