[Bug][SubscriptionBilling]: Invoicing Item on Subscription Package Line ignored for "Subscription Item" type items

[miljance]

Describe the issue

A bug has been identified in the Subscription & Recurring Billing module where the Invoicing Item No. specified on a Subscription Package Line is not correctly transferred to the resulting subscription line if the base item's Subscription Option is set to Subscription Item.

Currently, the system always uses the Subscription Item itself for invoicing and postings in this scenario. While it is acceptable for the Subscription Item to serve as the default Invoicing Item No., this should occur when no specific Invoicing Item No. has been defined in the Subscription Package. If a value is provided in the package, it must take precedence.

Expected behavior

The system should respect the following hierarchy when determining the invoicing item:

1. Primary: Use the Invoicing Item No. defined in the Subscription Package Line.
2. Fallback: Use the Subscription Item itself only if the Invoicing Item No. in the package is blank.

Steps to reproduce

1. Set up Items:

• Create Item A (Subscription Item) with Subscription Option = Subscription Item.
• Create Item B (intended Invoicing Item).

2. Define Subscription Package:

• Create a Subscription Package.
• On the Subscription Package Line, explicitly set the Invoicing Item No. to Item B.

3. Assign and Process:

• Assign the package to Item A.
• Create a subscription for Item A via Sales Order

4. Observe Result:

• Observed: The subscription line uses Item A for invoicing, ignoring the package configuration.
• Expected: The subscription line should use Item B.

Additional context

No response

I will provide a fix for a bug

• I will provide a fix for a bug