Add tests to improve coverage from 78% to 84% (gijzelaerr#642)

* Add tests to improve coverage from 78% to ~85%

Add test suites for untested code paths that don't require a real PLC:
- error.py: error routing, check_error(), error_wrap() decorator
- connection.py: socket mocking, COTP parsing, exception paths
- server/__main__.py: CLI entrypoint test
- s7protocol.py: response parser tests with crafted PDUs
- util/db.py: DB/Row type conversions, dict-like interface

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Fix test_server_cli.py when click is not installed

Use pytest.importorskip to gracefully skip CLI tests when click
(an optional dependency) is not available in the test environment.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: gijzelaerr