docs: update README with new SDK API documentation
- Rewrite all code examples for snake_case/Pydantic v2 API - Add AsyncOnspringClient documentation with async/await examples - Add error handling documentation with raise_for_status() examples - Update property names (statusCode -> status_code, etc.) - Update import paths (OnspringApiSdk -> onspring_api_sdk) - Update constructor parameter names (baseUrl -> url, apiKey -> key) - Add httpx as the underlying HTTP library
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