Add coverage enforcement to pytest comparison workflow

[JamesonRGrieve]

Summary

• collect pytest coverage for both the PR and target branches and capture it in workflow outputs
• parse coverage artifacts alongside existing test data and include them in artifacts for later jobs
• overhaul the comparison summary and notifications to report coverage deltas, regression counts, improvements, and new discovery warnings for every run

Testing

• no automated tests were run (not applicable)

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https://chatgpt.com/codex/tasks/task_e_68e3183079c8832eb40ce1c1236cf9a1