[PM-33473] Remove pm-29594-update-individual-subscription-page feature flag

[amorask-bitwarden]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-33473

📔 Objective

Remove the pm-29594-update-individual-subscription-page feature flag and its flagged code paths per the Feature Flags Lifecycle process.

Changes:

• Delete the PM29594_UpdateIndividualSubscriptionPage constant from Constants.cs.
• Strip three [RequireFeature(FeatureFlagKeys.PM29594_UpdateIndividualSubscriptionPage)] attributes from AccountBillingVNextController on GetSubscriptionAsync, ReinstateSubscriptionAsync, and UpdateSubscriptionStorageAsync — the vNext endpoints become unconditional for cloud.
• Delete cloud-only PostStorageAsync and PostReinstateAsync from AccountsController along with their tests.
• Delete AdjustStorageAsync and ReinstatePremiumAsync from UserService and IUserService along with their tests.
• Reduce AccountsController.GetSubscriptionAsync to a self-hosted-only endpoint ([SelfHosted(SelfHostedOnly = true)]) because SelfHostedAccountSubscriptionComponent on the clients side still needs it — the vNext replacement is cloud-only ([SelfHosted(NotSelfHostedOnly = true)]). The response model SubscriptionResponseModel is simplified to only the fields self-hosted requires: StorageName, StorageGb, MaxStorageGb, Expiration.
• Extract three shared types (BillingCustomerDiscount, BillingSubscription, BillingSubscriptionUpcomingInvoice) from the former SubscriptionResponseModel.cs into standalone files under src/Api/Models/Response/ in the same namespace. OrganizationResponseModel.cs still consumes them via using Bit.Api.Models.Response; and needs no changes.

Paired with the companion clients PR that removes the flag + dead code on the client side.

📸 Screenshots

N/A — no UI changes.

[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

[github-actions]

Checkmarx One – Scan Summary & Details – a9223b5f-bb61-4a79-8314-a757fa2bedb6

Great job! No new security vulnerabilities introduced in this pull request

[codecov]

Codecov Report

❌ Patch coverage is 0% with 73 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.01%. Comparing base (48060b0) to head (e7f7a50).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Api/Models/Response/BillingSubscription.cs	0.00%	49 Missing ⚠️
src/Api/Models/Response/BillingCustomerDiscount.cs	0.00%	14 Missing ⚠️
...els/Response/BillingSubscriptionUpcomingInvoice.cs	0.00%	7 Missing ⚠️
src/Api/Billing/Controllers/AccountsController.cs	0.00%	2 Missing ⚠️
...c/Api/Models/Response/SubscriptionResponseModel.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7496      +/-   ##
==========================================
- Coverage   59.10%   59.01%   -0.10%     
==========================================
  Files        2078     2081       +3     
  Lines       91526    91428      -98     
  Branches     8141     8122      -19     
==========================================
- Hits        54100    53952     -148     
- Misses      35500    35554      +54     
+ Partials     1926     1922       -4     

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