[PM-39925] feat: scaffold the Bit.Subscription.* and Bit.Invoicing libraries for invoice preview

[kdenney]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-39925

📔 Objective

Sets up the library skeleton for the preview-driven cart work. There is no user-facing behavior yet: the endpoint groups map no routes, and everything sits behind a feature flag that defaults off.

• Adds the PM36631_PreviewDrivenCart feature flag.
• Scaffolds three libraries under src/Libraries/: Bit.Invoicing (platform tier, which will own the invoice-preview projection) plus two feature-tier endpoint-group shells, Bit.Subscriptions.User and Bit.Subscriptions.Organization.
• Registers the feature libraries in the API host cloud-only (inside a !globalSettings.SelfHosted block). The host owns the route prefixes: the user group mounts at /account/billing/subscription/premium and the organization group at /organizations/{organizationId:guid}/billing/subscription.
• Each group attaches its shared cross-cutting chain (tags, the Application authorization policy, exception handling, and the PM36631_PreviewDrivenCart gate) to an empty group but maps no endpoints yet; those arrive in later slices. Only an authenticated caller is required at the group level, and each future handler will perform its own organization billing authorization check.
• Adds the new projects to the solution, a README per library, and metadata-based endpoint tests that assert the group conventions against a throwaway probe endpoint.
• Un-ignores the Subscriptions.User directory that the stock *.user gitignore pattern was silently hiding.

All three libraries reference Core as a documented deviation from the target billing-library contract (pending Bit.Integrations.Billing, per ADR-0032); each library's README records what it takes from Core.

This branch is stacked on the Stripe SDK upgrade work (PM-36225), which is its base and should merge first.

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

Reviewed the scaffolding for Bit.Invoicing, Bit.Subscriptions.User, and Bit.Subscriptions.Organization: project/solution wiring, Startup.cs registration and endpoint mapping, the PM36631_PreviewDrivenCart flag ownership via [FlagKeyCollection], the endpoint-group convention tests, CODEOWNERS, the .gitignore re-inclusion, and the writing-server-code skill addition. The endpoint groups map no routes and the flag defaults off, so there is no user-facing behavior change; the internal group name correctly keeps future endpoints out of api.public.json, and AddOpenApiEndpointDataSource is a no-op outside swagger generation so there is no double route registration with UseEndpoints. Lock file changes are project-reference entries only — no new NuGet packages and no version bumps. No findings met the bar for an inline comment.

Code Review Details

No findings. Prior review threads on the group name, placeholder tests, the Core feature-flag dependency, and the self-host service-registration guard have all been addressed in the current head.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.31%. Comparing base (ac309aa) to head (a47db11).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8208      +/-   ##
==========================================
+ Coverage   63.29%   63.31%   +0.01%     
==========================================
  Files        2401     2406       +5     
  Lines      104043   104091      +48     
  Branches     9426     9429       +3     
==========================================
+ Hits        65857    65904      +47     
- Misses      35930    35931       +1     
  Partials     2256     2256              

☔ 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.