[PM-38422] feat: Stack customer-level discount with churn and migration discounts

[amorask-bitwarden]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-38422

📔 Objective

When a churn-mitigation, proactive, or milestone-migration coupon is applied to a subscription that already carries a customer-level discount (customer.discount), the new coupon was written into the subscription's (or schedule phase's) discounts array on its own. Because Stripe lets a subscription/phase-level discount override the customer-level one, the pre-existing customer discount was silently dropped from billing.

This adds a shared DiscountExtensions.MergeDiscountCouponIds extension (customer coupon first, deduped by id with StringComparison.Ordinal, order-preserving) that merges the customer-level discount with the existing subscription/phase discounts and the newly applied coupon so they stack. It is applied wherever a discounts array is written and a pre-existing customer discount could be shadowed:

• Churn-mitigation redeem (churn-only subscription update + migration-cohort Phase 2)
• Premium / families / business price-increase scheduler Phase 2 builders
• Premium-storage and billing-address schedule rebuilds, and the worker UpcomingInvoiceHandler automatic-tax rebuild — future phases only (active phases are mirrored verbatim so a discount is not re-applied on the already-billed period)
• Org subscription-update schedule rebuild — future phase only
• GetBitwardenSubscriptionQuery dedups the carried coupon so the tax preview does not double-count it

The customer coupon is carried regardless of Coupon.Valid (an active customer.discount is one Stripe is already applying). Unblocks migration cohorts that include customers with pre-existing discounts. Unit tests cover the helper, the stacking/repro/self-heal cases, and the active-vs-future-phase boundary at each site.

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

Reviewed this billing change that stacks a pre-existing customer-level discount (customer.discount) with newly applied churn, proactive, and milestone-migration coupons, rather than letting the subscription/phase-level discount silently shadow it. The new DiscountExtensions.MergeDiscountCouponIds helper merges customer-first, deduplicates by id with StringComparison.Ordinal, and is null/empty-safe; it is applied consistently across the redeem, scheduler, storage, billing-address, and worker-rebuild sites, gating carry-over to future phases (StartDate > now) so a discount is never re-applied on the already-billed active period. I traced the active-vs-future phase boundary, the merged-set self-heal no-op guard, and the DistinctBy dedup in the tax-preview query, and verified each against its unit tests.

Code Review Details

No findings. The change is well-structured with comprehensive unit coverage:

• Helper edge cases (null/empty interleaving, ordinal case-sensitivity, customer-first ordering, invalid-coupon carry-through, cross-source dedup).
• Active-vs-future phase boundary asserted at each call site, including the consumed-phase suppression path.
• Self-heal-on-retry and true-no-op (preserves ChurnDiscountAppliedDate) cases for the churn redeem flow.
• Tax-preview dedup so a coupon present on both the customer and the mirrored Phase 2 list is passed once.

Null-safety (customerDiscount?.Coupon?.Id, existingDiscountCouponIds ?? [], IsNullOrEmpty guards, the coupon is not null filter before DistinctBy) is handled throughout. The validity-gating difference between the merge helper (carries an active customer.discount regardless of Coupon.Valid) and the tax-preview query (which filters on validity) affects only the displayed estimate and matches the pre-existing behavior of that path.

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.4% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 97.29730% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.23%. Comparing base (19cb812) to head (18fd786).

Files with missing lines	Patch %	Lines
...tion/Commands/RedeemChurnMitigationOfferCommand.cs	85.71%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7831      +/-   ##
==========================================
+ Coverage   61.22%   61.23%   +0.01%     
==========================================
  Files        2209     2209              
  Lines       97716    97723       +7     
  Branches     8815     8818       +3     
==========================================
+ Hits        59824    59841      +17     
+ Misses      35768    35760       -8     
+ Partials     2124     2122       -2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[kdenney]

🔥 Very clean design; nice!