test: cover issue 1081

[brentthomas248]

Closes #1081

Summary:

• strengthen the GET /v1/feature-flags e2e test to assert the complete primary endpoint payload
• clean up all feature flag env overrides used by the endpoint between tests
• correct the stale invalid-env e2e value and type the response/context helpers so changed-file lint passes

Tests:

• npm run test:e2e --prefix backend -- feature-flags.e2e-spec.ts --runInBand
• npm test --prefix backend -- feature-flags.service.spec.ts --runInBand
• cd backend && npx --no-install eslint test/feature-flags.e2e-spec.ts
• git diff --check -- backend/test/feature-flags.e2e-spec.ts

Note:

• npm run test:backend is currently blocked by unrelated baseline failures outside this change: src/social-link/social-links.controller.spec.ts has a TS1128 syntax error at line 293, several unrelated controller specs are missing ThrottlerGuard module options, and unrelated throttle/Swagger metadata expectations fail.