Fix CI pipeline to publish coverage

[SaridakisStamatisChristos]

Summary

• install the project test dependencies in CI before executing the suite
• run pytest with coverage reporting enabled so that Codecov can find coverage data
• emit an XML coverage report by default so the Codecov uploader succeeds

Testing

• python -m pytest --cov=kll_sketch --cov-report=term-missing --cov-report=xml kll_sketch/tests (fails: pytest-cov missing in the offline environment)

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https://chatgpt.com/codex/tasks/task_e_68e5073f9fa883338abe05f8991012d8

[codecov-commenter]

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