TSM-05: convert base service hydration layer to TypeScript

[Justin Hammond (Justintime50)]

Summary

• rename base hydration files to TypeScript: base_service and easypost_object
• preserve dynamic model hydration and Symbol.hasInstance behavior unchanged
• keep migration permissive with temporary ts-nocheck on converted files

Validation

• npm run typescript
• npm run build
• npx eslint src/services/base_service.ts src/models/easypost_object.ts
• npx vitest run test/services/module_exports_compat.test.js

Notes

• targeted service tests that construct the client require EASYPOST_TEST_API_KEY in local env