Prevent duplicate payment

[Kaylahray]

Prevent duplicate payments for the same invoice

Closes #89

What changed

• lib.rs — record_payment already contains an idempotency guard at step 4
  (has_payment check → ContractError::PaymentAlreadyRecorded). No logic change
  needed; the guard was verified to fire before any state mutation or event emission.

• test.rs — Added three new acceptance-criteria tests under the section
  // Prevent duplicate payments — acceptance-criteria tests:

  Test	What it proves
  test_first_payment_succeeds_emits_event_and_increments_count	Happy path: first record_payment stores the record, emits invoice_payment_recorded, and increments payment_count to 1.
  test_duplicate_payment_fails_no_event_count_unchanged	Duplicate: second call with the same invoice_id reverts with PaymentAlreadyRecorded, emits no event, and leaves payment_count unchanged.
  test_cross_asset_duplicate_same_invoice_id_fails	Cross-asset: same invoice_id with a different asset (XLM → USDC) also reverts with PaymentAlreadyRecorded — invoice_id is the sole uniqueness key.

Acceptance criteria satisfied

• ✅ Duplicate invoice_id reliably reverts with ContractError::PaymentAlreadyRecorded
• ✅ No payment_recorded event emitted on failure; success path unchanged
• ✅ State (count, storage) is not mutated on a failed duplicate attempt
• ✅ Cross-asset replay with same invoice_id is also rejected
• ✅ No additional storage writes on failed duplicates — gas unaffected

[Cedarich]

LGTM