Update codecov/codecov-action action to v6 (#741) #2399

Job	Run time
ruff-lint	7s
uv-check	7s
pymarkdown	8s
flake8	11s
pre-commit / autofix-pre-commit	2m 1s
mypy (package)	30s
pytest	17s
mypy (tests)	18s
build-and-publish	51s
release	0s
	4m 30s

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