feat: align with KSeF API 2.6.1

[smekcio]

Summary

• refresh the committed KSeF OpenAPI snapshot to API 2.6.1 metadata and updated CIRFMF/ksef-api documentation links
• update public compatibility wording in README and changelog
• add regression coverage for the OpenAPI snapshot metadata
• make CLI secret-source warning detection robust across Click/Typer ParameterSource enum implementations
• declare click in dev requirements because CLI tests import click directly

Verification

• python tools/sync_generated.py --check --no-fallback
• python -m pytest tests/test_generated_artifacts.py tests/test_tools_sync_generated.py
• python -m pytest tests/test_http.py tests/test_clients.py tests/test_services_batch.py
• python -m pytest
• python -m ruff check src/ksef_client/cli/commands/auth_cmd.py tests/test_generated_artifacts.py