[Extensibility Request][Subscription Billing] Move key page variables to protected var to enable PageExtension access

Describe the issue

Page 8002 "Extend Contract" currently exposes only a limited set of variables in its
protected var section, which was recently extended to include ItemNo, VariantCode,
QuantityDecimal, ExtendCustomerContract, ExtendVendorContract, and SellToCustomerNo.
However, several additional variables that partner extensions need to read or write remain
in the global (unprotected) var block and are therefore inaccessible from a PageExtension.

This prevents partners from implementing key automation scenarios on top of the standard
"Extend Contract" flow without having to clone the entire page — which in turn is blocked
by the internal procedures already addressed in issue #8385 / pull request #8387.

The following variables should be moved from the global var block to protected var:

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1. CustomerContractNo: Code[20]

Why needed: A PageExtension needs to pre-populate the customer contract number based
on context from the calling page (e.g. a usage data import line that already contains a
resolved contract ID). Without protected var access, the value cannot be set from a
PageExtension after the page is opened.

---

2. VendorContractNo: Code[20]

Why needed: When the "Extend Contract" page is opened from a customer subscription
contract that has a linked vendor contract, a PageExtension should automatically pre-fill
the vendor contract number. This is not possible as long as VendorContractNo remains in
the unprotected var block.

---

3. SubscriptionDescription: Text[100]

Why needed: When a user selects an item variant on the "Extend Contract" page, the
subscription description should be updated to reflect the variant's description. A
PageExtension cannot write to SubscriptionDescription to implement this behaviour
because the variable is not in protected var.

---

4. ItemDescription: Text[100]

Why needed: Closely related to SubscriptionDescription — a PageExtension needs to
update the item description when the variant code changes. Currently inaccessible for
the same reason.

---

5. UnitPrice: Decimal

Why needed: Partner extensions that derive the unit price from an external source
(e.g. a vendor invoice line or a distributor price list) need to write the resolved
price into UnitPrice before the contract extension is executed. This is not possible
without protected var access.

---

6. UnitCostLCY: Decimal

Why needed: Same as UnitPrice — the unit cost in LCY needs to be settable from a
PageExtension to support scenarios where cost data is available from an upstream record
(e.g. a purchase invoice line matched to a vendor subscription contract).

---

7. ProvisionStartDate: Date

Why needed: When the "Extend Contract" page is opened programmatically from an
automation extension (e.g. a usage data import batch), the provision start date is
already known from the import record. A PageExtension should be able to set this date
directly. Currently only settable via SetParameters(), which requires the page to be
called in a specific way — a PageExtension on OnOpenPage cannot override it.

---

Current state (BC 28-vNext / BC 29-vNext)

The protected var section currently contains:

protected var
    ItemNo: Code[20];
    VariantCode: Code[10];
    QuantityDecimal: Decimal;
    ExtendCustomerContract: Boolean;
    ExtendVendorContract: Boolean;
    SellToCustomerNo: Code[20];

The seven variables listed above remain in the unprotected global var block.

Expected behavior

The seven variables are moved to the protected var section, making them writable
from PageExtensions:

protected var
    ItemNo: Code[20];
    VariantCode: Code[10];
    QuantityDecimal: Decimal;
    ExtendCustomerContract: Boolean;
    ExtendVendorContract: Boolean;
    SellToCustomerNo: Code[20];
    CustomerContractNo: Code[20];
    VendorContractNo: Code[20];
    SubscriptionDescription: Text[100];
    ItemDescription: Text[100];
    UnitPrice: Decimal;
    UnitCostLCY: Decimal;
    ProvisionStartDate: Date;

Steps to reproduce

Create a PageExtension on Page 8002 "Extend Contract" and attempt to assign a value to
any of the listed variables (e.g. CustomerContractNo := 'CC10000';). Result: AL0132
compiler error — "CC10000 is not accessible".

Additional context

This request is related to issue #8385 and pull request #8387, which addressed inaccessible
internal procedures on the same page. Together, these changes would allow partners to
build automation extensions on top of the standard "Extend Contract" flow without cloning
the page.

I will provide a fix for a bug

• I will provide a fix for a bug