Add comprehensive unit tests for Client API and model validation (#403)

* Add comprehensive unit tests for Client API and model validation

Increase unit test count from 19 to 106:

- test_client.py: 38 tests covering all Client API methods (alerts,
  blackouts, customers, heartbeats, keys, permissions, users, groups,
  auth, search/pagination, error handling) using requests_mock
- test_models.py: 49 tests covering model validation, defaults,
  parse error handling, tabular output, repr, and enum values for
  Alert, Blackout, Heartbeat, Group, ApiKey, Permission, User,
  Customer, Scope, ChangeType, and NoteType

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix flake8: remove unused imports in test files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Nick Satterly <nsa@softpay.io>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people