Incorrect price per unit displayed for Duo Pro and additional seats on uninvoiced subscriptions

Problem

When purchasing Duo Pro or additional base plan seats for a subscription that hasn't been invoiced yet (before bill run), the Customers Portal displays incorrect or unpredictable price per unit values. After the bill run is completed, the pricing displays correctly.

Evidence

From https://gitlab.com/gitlab-org/customers-gitlab-com/-/work_items/14566#note_2870568419

Before bill run	After bill run

Additional Context

• Discovered during testing of work item #14566+
• Related discussion: https://gitlab.com/gitlab-org/customers-gitlab-com/-/work_items/14566#note_2870476506
• May be related to issue #14429+ (similar pricing display issue)

Proposal

Investigate why price per unit calculations are incorrect for uninvoiced subscriptions and implement a fix to ensure accurate pricing is displayed before bill runs are executed.

Acceptance Criteria:

• Price per unit for Duo Pro purchases displays correctly before bill run
• Price per unit for additional base plan seat purchases displays correctly before bill run
• Pricing remains accurate after bill run completion
• Verify fix doesn't impact other subscription pricing displays