[PM-38830] fix: Surface fixed-amount and time-bounded discounts in billing API

[amorask-bitwarden]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-38830

📔 Objective

Fixed-amount (Stripe amount_off) churn-mitigation coupons — and repeating / time-bounded discounts — are billed correctly in Stripe but were never rendered in the Web Vault billing UI, because the server's billing contracts dropped the underlying values. This PR adds the missing data so the clients can format and display them. It is the server-side portion; a companion client PR consumes the new fields.

Churn offer (amount_off)

• Add AmountOff (decimal?, USD dollars) to ChurnMitigationOfferResult and map it from Stripe Coupon.AmountOff (minor units → dollars via / 100M) in GetChurnMitigationOfferQuery. null for percentage coupons.

Customer discount (time-bounded)

• Add End (DateTime?) and DurationInMonths (long?) to both the domain SubscriptionInfo.BillingCustomerDiscount and the API BillingCustomerDiscount response model, populated in both the Discount and Coupon constructors. These let the subscription page apply/show repeating discounts (e.g. the 10% × 12-month monthly-plan churn case) instead of suppressing them.

All new fields are additive and nullable — safe for the V±2 client matrix. The existing Active flag is intentionally left unchanged (it keeps its "perpetual / no end date" meaning; other consumers such as sm-subscribe.component.ts still depend on it). No feature-flag, DI, or migration changes. Unit tests cover the cents→dollars conversion, amount + duration coexistence, and an explicit Active-unchanged regression guard.

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

Reviewed the server-side billing contract changes that surface fixed-amount (amount_off) churn-mitigation coupons and time-bounded (repeating) discounts. The change adds AmountOff to ChurnMitigationOfferResult and End / DurationInMonths to both the domain SubscriptionInfo.BillingCustomerDiscount and the API response model. All new fields are additive and nullable, the cents→dollars conversions are correct and consistent with existing helpers, and the intentionally-unchanged Active semantics are documented and guarded by regression tests.

Code Review Details

No findings. The change is well-scoped, additive, and thoroughly tested:

• Conversions verified: coupon.AmountOff.ToMajor() (1500 → 15.00) is functionally equivalent to the existing ConvertFromStripeMinorUnits divisor.
• No breaking changes — new fields are nullable and safe for the V±2 client matrix; the single production construction site uses named arguments.
• No security or zero-knowledge concerns — only Stripe billing metadata, no PII or vault data.
• Test coverage spans cents→dollars mapping, amount/duration coexistence, and explicit Active-unchanged guards in both the domain and API layers.

The earlier suggestion to reuse CurrencyExtensions.ToMajor() was already addressed in commit 41bdf1d.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.28%. Comparing base (2f56f5d) to head (2d0b759).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7814      +/-   ##
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+ Coverage   61.26%   61.28%   +0.02%     
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  Files        2194     2194              
  Lines       97313    97325      +12     
  Branches     8768     8769       +1     
==========================================
+ Hits        59615    59646      +31     
+ Misses      35584    35565      -19     
  Partials     2114     2114              

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

[cyprain-okeke]

Nice Work