ci: run tests with pytest discovery

[Pigbibi]

Summary

• install pytest in CI
• run tests with pytest discovery so top-level pytest modules are included

Verification

• PYTHONPATH=.:../QuantPlatformKit/src:../UsEquityStrategies/src .venv/bin/python -m pytest -q tests/test_order_alerts.py tests/test_profile_key_governance.py tests/test_runtime_bootstrap_adapters.py tests/test_runtime_broker_adapters.py tests/test_runtime_composer.py tests/test_runtime_notification_adapters.py tests/test_runtime_reporting_adapters.py tests/test_runtime_strategy_adapters.py
• PYTHONPATH=.:../QuantPlatformKit/src:../UsEquityStrategies/src .venv/bin/python -m pytest -q tests