fix(test): add importOriginal to stripe-client vi.mock — restores integration tests

[EmmaLouise2018]

Closes #3548.

Two integration tests vi.mock src/billing/stripe-client.js but the mock factories don't export listCustomersWithOrgIds. Server startup calls OrganizationDatabase.syncStripeCustomers which uses that method, throws at HTTPServer.start, breaks every integration test booting through these mocks.

Stack at fail time:

[vitest] No "listCustomersWithOrgIds" export is defined on
"../../src/billing/stripe-client.js" mock
  at OrganizationDatabase.syncStripeCustomers (server/src/db/organization-db.ts:1708:29)
  at HTTPServer.start (server/src/http.ts:8591:21)
  at server/tests/integration/content-my-content.test.ts:137:5
  at server/tests/integration/admin-endpoints.test.ts:62:5

Fix

Switch the two vi.mock factories to vitest's importOriginal pattern. Partial mocks now inherit real stripe-client exports — listCustomersWithOrgIds and every future method flow through automatically without per-PR mock churn. Strictly better than the previous explicit-export pattern.

Files

• server/tests/integration/content-my-content.test.ts:69
• server/tests/integration/admin-endpoints.test.ts:28

Test plan

• npx vitest run server/tests/integration/content-my-content.test.ts server/tests/integration/admin-endpoints.test.ts --config server/vitest.config.ts — mock failure resolved locally; tests then short-circuit on ECONNREFUSED 127.0.0.1:53198 because local Postgres is unavailable in this worktree. CI has Postgres and will run the full suite.
• tsc --noEmit -p server/tsconfig.json — clean (also verified by repo typecheck precommit hook).
• Repo precommit (test:unit, test:test-dynamic-imports, typecheck) — all green.

Unblocks

• fix(registry): flip type==='buying' filter inversions to 'sales' + add sales to by_type tally #3540 integration-tests job (was failing on this stack).
• Any future integration test that boots the server through either mock.