feat(dimensional): add tax line-item capture to tfact_order

[blarghmatey]

Context

PR #1868 added tfact_order covering order totals. Reviewer noted that tax amounts should be captured when a tax was applied.

What

Add tax information to tfact_order once source staging models expose tax line items:

• Identify the tax line-item source in MITx Online and xPro order systems
• Add staging model(s) if not already present
• Add order_tax_amount, order_tax_rate, and/or order_pre_tax_price columns to tfact_order

Dependencies

• Requires investigation of tax data availability in stg__mitxonline__app__postgres__ecommerce_* and stg__mitxpro__app__postgres__ecommerce_* sources
• May require new intermediate models

Acceptance Criteria

• Tax source tables identified and documented
• Staging/intermediate models created if needed
• tfact_order updated with tax columns
• _fact_tables.yml updated with column descriptions and tests