Add system pytest and use it instead

[adamoutler]

📌 Description

Fix failing unit tests in GitHub CI by installing pytest into the system environment inside the test container. The CI job spins up the container and uses the container's system Python (not the repository venv), so installing pytest system-wide ensures the test runner is available and the job behaves the same as the devcontainer.

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🔍 Related Issues

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📋 Type of Change

Please check the relevant option(s):

• 🐛 Bug fix
• 🧪 Test addition or change
• 🔧 Build/config update
• 🔨 CI/CD or automation
• ✨ New feature
• ♻️ Code refactor
• 📚 Documentation update
• 🚀 Performance improvement
• 🧹 Cleanup / chore

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📷 Screenshots or Logs (if applicable)

See changes made in #1420

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🧪 Testing Steps

• Push branch and allow GitHub Actions to run the docker-based test job.
• Verify the job installs pytest inside the container (e.g., pip install pytest runs in the job log).
• Verify pytest is invoked from the container system Python and that tests complete successfully.
• (Local verification) Build the test image and run a container, then inside it run pip install pytest && pytest -q to confirm behavior matches CI.

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✅ Checklist

• I have read the Contribution Guidelines
• I have tested my changes locally
• I have updated relevant documentation (if applicable)
• I have verified my changes do not break existing behavior
• I am willing to respond to requested changes and feedback

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🙋 Additional Notes

This change mirrors the devcontainer testing environment and prevents CI from failing when pytest is not available on the container's system interpreter. Consider pinning the pytest version in a follow-up to avoid unexpected test runner upgrades.