Task 6: Increase test coverage across all endpoints

[godely]

Task 6: Increase test coverage across all endpoints

• Write unit tests for all currently untested endpoint modules: daily_readiness, daily_spo2, sleep_time, rest_mode_period, daily_stress, daily_resilience, daily_cardiovascular_age, vo2_max, ring_configuration, personal, session, tag, enhanced_tag, and workout.
• Cover success, failure, and raw‐response (return_model=False) paths, aiming for ≥90% coverage in oura_api_client/api/ and oura_api_client/models/.