[main] bug 643971 - Implement checks for blocked customers and vendors in advance letters

[DavidHolusa]

What & why

Add blocked vendor/customer validation to advance letter headers and posting. When a vendor or customer is blocked (All, Payment for vendors; All, Ship, Invoice for customers) or privacy-blocked, the system now raises an error when:

Setting the vendor/customer on a purchase/sales advance letter header (validation).
Posting an advance letter payment (post restrictions check).
This aligns advance letter behavior with standard purchase/sales document blocking and prevents users from creating or posting advance letters for blocked parties.

Linked work

Fixes AB#643971

How I validated this

• I read the full diff and it contains only changes I intended.
• I built the affected app(s) locally with no new analyzer warnings.
• I ran the change in Business Central and confirmed it behaves as expected.
• I added or updated tests for the new behavior, or explained below why none are needed.

What I tested and the outcome

App changes (4 files):

CustomerCZZ.TableExt.al — added CheckBlockedCustOnAdvanceLettersCZZ(Transaction) that checks Privacy Blocked and Blocked <> " ".
VendorCZZ.TableExt.al — added CheckBlockedVendOnAdvanceLettersCZZ(Transaction) with the same logic for vendors.
SalesAdvLetterHeaderCZZ.Table.al — calls the customer check on "Bill-to Customer No." validation (Transaction=false) and in CheckSalesAdvanceLetterPostRestrictions (Transaction=true).
PurchAdvLetterHeaderCZZ.Table.al — calls the vendor check on "Pay-to Vendor No." validation (Transaction=false) and in CheckPurchaseAdvanceLetterPostRestrictions (Transaction=true).
Test changes (2 files, 14 new tests):

Purchase Advance Payments (codeunit 148108) — 6 tests:

CreatePurchAdvLetterWithVendorBlockedAll — error on validation with Blocked::All
CreatePurchAdvLetterWithVendorBlockedPayment — error on validation with Blocked::Payment
CreatePurchAdvLetterWithPrivacyBlockedVendor — error on validation with Privacy Blocked
PostPaymentPurchAdvLetterWithVendorBlockedAll — error on posting with Blocked::All
PostPaymentPurchAdvLetterWithVendorBlockedPayment — error on posting with Blocked::Payment
PostPaymentPurchAdvLetterWithPrivacyBlockedVendor — error on posting with Privacy Blocked

Sales Advance Payments (codeunit 148109) — 8 tests:

CreateSalesAdvLetterWithCustomerBlockedAll — error on validation with Blocked::All
CreateSalesAdvLetterWithCustomerBlockedShip — error on validation with Blocked::Ship
CreateSalesAdvLetterWithCustomerBlockedInvoice — error on validation with Blocked::Invoice
CreateSalesAdvLetterWithPrivacyBlockedCustomer — error on validation with Privacy Blocked
PostPaymentSalesAdvLetterWithCustomerBlockedAll — error on posting with Blocked::All
PostPaymentSalesAdvLetterWithCustomerBlockedShip — error on posting with Blocked::Ship
PostPaymentSalesAdvLetterWithCustomerBlockedInvoice — error on posting with Blocked::Invoice
PostPaymentSalesAdvLetterWithPrivacyBlockedCustomer — error on posting with Privacy Blocked

All tests verify the exact error message text. Helper functions CreateBlockedVendor/CreateBlockedCustomer and shared base procedures reduce duplication.

Risk & compatibility

This is a stricter validation — advance letters previously allowed blocked vendors/customers. Existing advance letters with blocked parties will fail on subsequent payment posting, which is the intended behavior matching standard document blocking.