chore: remove deprecated coupons/codes feature and all ecommerce calls

[macdiesel]

Summary

Removes the deprecated "codes" / "coupon codes" / "coupons" feature from the admin portal frontend and eliminates all calls to the ecommerce backend (EcommerceApiService).

What was removed

Components/Containers (deleted entirely):

• src/components/CodeAssignmentModal/
• src/components/CodeManagement/
• src/components/CodeModal/
• src/components/CodeReminderModal/
• src/components/CodeRevokeModal/
• src/components/CodeSearchResults/
• src/components/Coupon/
• src/components/CouponDetails/
• src/components/RequestCodesPage/
• src/components/CodeDeprecationAlert/
• src/components/EmailTemplateForm/
• src/components/TemplateSourceFields/
• src/containers/CodeAssignmentModal/
• src/containers/CodeReminderModal/
• src/containers/CodeRevokeModal/
• src/containers/CouponDetails/
• src/containers/SaveTemplateButton/
• src/containers/TemplateSourceFields/

Services:

• src/data/services/EcommerceApiService.js — all ecommerce API calls removed

Redux slices:

• src/data/actions/coupons.js
• src/data/actions/codeAssignment.js
• src/data/actions/codeReminder.js
• src/data/actions/codeRevoke.js
• src/data/actions/emailTemplate.js
• src/data/reducers/coupons.js
• src/data/reducers/emailTemplate.js
• src/data/constants/emailTemplate.js

Hooks:

• useEnterpriseOffer hook and its test
• useCoupons hook from EnterpriseSubsidiesContext
• useApplicableCoupons from subsidy-request-modals
• useSubsidySummaryAnalyticsApi no longer fetches for ecommerce budget type

Other:

• NoAvailableCodesBanner component
• ApproveCouponCodeRequestModal component
• BUDGET_TYPES.ecommerce constant
• ROUTE_NAMES.codeManagement constant
• SUBSIDY_TYPES.coupon and SUPPORTED_SUBSIDY_TYPES.coupon constants

Eliminates all calls to the ecommerce backend from this frontend.

Behavior Changes

• Routes removed: /admin/coupons/* and /admin/coupons/request-codes no longer exist
• Sidebar: "Code Management" menu entry removed
• Context values no longer provided: coupons is no longer in EnterpriseSubsidiesContext
• Budget IDs: useBudgetId now always treats the budgetId as a subsidyAccessPolicyId (UUID); enterprise offer integer IDs are no longer supported
• Feature flags: enableCodeManagementScreen / enablePortalCodeManagementScreen props removed from component tree
• Subsidy types: coupon no longer appears as a subsidy type option in settings or subsidy request flows

Risks / Things Reviewers Should Verify

• Verify no enterprise customer still relies on the legacy coupon UI (backend should have migrated all to learner credit)
• Verify learner-credit budgets still render correctly without the enterpriseOffers context value
• Verify subsidy request configuration creation still works (now only checks for subscriptions, not coupons)
• The subsidyRequestsCounts.couponCodes value still exists in subsidy-requests hook (populated from enterprise-access API, not ecommerce) — sidebar no longer displays it

Notes

• Imports that previously referenced @edx/frontend-enterprise-utils continue to use @2uinc/frontend-enterprise-utils per edx-fork convention.
• All 365 test suites pass (2503 tests passing, 42 skipped)
• ESLint passes cleanly
• Opened as draft for team review.

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.32%. Comparing base (6fb5654) to head (922033e).

Files with missing lines	Patch %	Lines
...rner-credit-management/BudgetDetailRedemptions.jsx	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   86.55%   87.32%   +0.76%     
==========================================
  Files         822      759      -63     
  Lines       19436    17622    -1814     
  Branches     4029     3675     -354     
==========================================
- Hits        16823    15388    -1435     
+ Misses       2549     2146     -403     
- Partials       64       88      +24     

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