feat(api): move SDK to typed models and harden OpenAPI tooling

[smekcio]

Summary

• move the public SDK from dict-based request and response handling to typed OpenAPI-backed models across clients, workflows, CLI adapters, and exceptions
• add OpenAPI tooling that downloads the live KSeF schema, validates generated models in CI, and falls back to the last committed snapshot when the endpoint is temporarily unavailable
• simplify the public integration path with certificate-selection and invoice-query helpers, then align README, workflows, and runnable examples to the lower-ceremony typed API

Testing

• python tools/lint.py
• python -m pytest -q
• python tools/generate_openapi_models.py --check --output src/ksef_client/openapi_models.py
• python tools/check_coverage.py --src src/ksef_client/clients