Enhance the AI Billing Agent to support bundled plans with different line items and tax rates

🔍 Problem Statement

As a publisher, I cannot create a subscription that invoices as separate line items taxed under different rules, breaking legally required per-item tax display.

💡 User Story

1. As a publisher, I want to describe a bundled plan's components and their tax rates in the AI Plan Assistant, so that each component bills as its own line item.

2. As a publisher, I want to be able to define rules that exclude taxes if a VAT number for the buyer is provided or apply defined rates for certain countries even when VAT number is provided.

3. As a subscriber, I want checkout to show each component's price and tax before I pay, so that the quoted total matches the invoiced total.

🎯 Definition of Done (DoD)

A feature is done when:

✔ Given a publisher describes components (print + digital + data center etc) in the AI Plan Assistant, when the plan is saved, then the components persist as a JSON object on the plan.

✔ Tax rules should be defined for each AI-created plan, allowing admins to configure different tax rules for each line item within bundled products in the prompt.

Example: “Create an annual bundle at €120 EUR — €72 for Loan Libraries Digital and €48 for Loan Libraries Print. Apply a 23% VAT rate to Loan Libraries Digital and a 5% VAT rate to Loan Libraries Print.”

✔ Admins can also define invoice note rules in the AI prompt. For example, if a VAT ID is provided and the subscriber’s address is in Germany, apply 7% VAT to print products and add the following invoice note:

“Tax-exempt intra-Community supply pursuant to § 4 No. 1 lit. b in conjunction with § 6a German VAT Act. No German VAT is charged.”

✔ The AI engine should interpret these rules and apply the appropriate VAT rate to each line item based on the subscriber’s address and the product type.

✔ Given a described component set, when the assistant validates it, then it blocks save unless amounts sum to the plan total, share one currency, and each carries a description and other validations already exist on the AI Plan Assistant.

✔ Given validation passes, when the assistant asks for confirmation, then no plan is written until the publisher explicitly confirms.

✔ Given a component plan, when Open API returns checkout data, then each component's description, amount and resolved tax are included before purchase. Conditions like: when a client's VAT number is provided, either exclude taxes or apply certain rates per country should be covered too.

✔ Given a subscription on a component plan, when it is created or renewed, then the subscription engine invokes the AI Billing Agent and the invoice carries one line item per component.

✔ Given a component line item, when tax is resolved, then the rate derives from the component's tax category and the buyer's shipping address, not from a rate preconfigured on the plan.

✔No changes to the existing refund and accounting logic will be introduced as part of this iteration.