refactor: add creator param not-found helper

[eliterdav09-creator]

Fixes #46

Summary

• add sendCreatorParamNotFound as the reusable creator-param not-found shortcut
• adopt it in the creator metadata route instead of spelling out the generic resource helper there
• add focused response-helper coverage for the exact 404 payload

Test plan

• pnpm test -- src/utils/test/api-response.utils.test.ts --runInBand
• pnpm exec prisma generate && pnpm exec tsc --noEmit
• pnpm lint -- src/utils/api-response.utils.ts src/utils/test/api-response.utils.test.ts src/modules/admin/admin.controllers.ts