ci: measure coverage on the unit suite too

[AlexeyShalaev]

codecov/patch fails on any pull request that touches code outside the Redis repository, and it is not the pull request's fault.

Coverage is collected in one place: the integration job runs pytest -m integration --cov=idempotency_kit and uploads that report. The unit matrix runs pytest -m unit with no coverage at all. So everything exercised only by unit tests reads as uncovered to Codecov, and a diff made mostly of unit-covered code arrives as a red patch status. The unit suite on its own reaches 95.66% of the package, none of which Codecov has ever seen.

This adds --cov to the unit run and uploads it from the 3.13 matrix entry, with flags: unit on that upload and flags: integration on the existing one so Codecov merges the two reports rather than letting the second overwrite the first. Nothing else changes: the integration job keeps its own --cov-fail-under=50, the unit job keeps the 85% floor that pyproject.toml already sets, and the upload stays continue-on-error so Codecov being down cannot fail a build.

codecov/patch is not the required status check — All checks passed is — so this was never blocking a merge. It was making every pull request look broken.

Found while fixing the findings in #22, which is where the red patch status shows. That branch turns green once it is updated on top of this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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