Implement App model and three endpoint methods: list_apps (paginated),
get_app (by ID), and batch_get_apps (up to 100 by IDs). Add
comprehensive tests for success, pagination, not-found, and batch
scenarios.
Implement ping() method on OnspringClient. Set up wiremock-based test
infrastructure with shared setup helper. Add tests for success and
unauthorized scenarios verifying correct headers are sent.
Set up Cargo.toml with dependencies (reqwest, serde, thiserror, tokio,
uuid, chrono). Implement OnspringClient with builder pattern and HTTP
helper methods. Define error types, enums, and paging models.