Coverage on CI is failing to report coverage

Start with the CI workflow that runs coverage and inspect the linked GitHub Actions job, especially the coverage combine, html, and report commands. Trace why coverage data is absent or skipped for some Python versions, then verify that coverage is generated, reported, and that reporting errors fail the job instead of being ignored.

1. Since https://github.com/ipython/ipykernel/pull/1358 the coverage gets skipped as some Python versions fail
2. Even before it does not report the coverage correctly, for example at https://github.com/ipython/ipykernel/commit/90133b7ac4801fa2c192c86c76982aa3a086896a it shows " No data to report."

and there are some errors which do not fail the job:

https://github.com/ipython/ipykernel/actions/runs/13475482660/job/37654246364#step:3:130

Artifact coverage-data.zip successfully finalized. Artifact ID 2635491504
Artifact coverage-data has been successfully uploaded! Final size is 523720 bytes. Artifact ID is 2635491504
Artifact download URL: https://github.com/ipython/ipykernel/actions/runs/13475482660/artifacts/2635491504
The 9 artifact(s) have been successfully merged!
Artifact 'coverage-3556' (ID: 2635490320) deleted
Artifact 'coverage-124[93](https://github.com/ipython/ipykernel/actions/runs/13475482660/job/37654246364#step:3:98)' (ID: 2635485998) deleted
Artifact 'coverage-21348' (ID: 2635486680) deleted
Artifact 'coverage-7093' (ID: 2635486884) deleted
Artifact 'coverage-14416' (ID: 2635491187) deleted
Artifact 'coverage-1062' (ID: 2635487118) deleted
Artifact 'coverage-8930' (ID: 2635486042) deleted
Artifact 'coverage-15[94](https://github.com/ipython/ipykernel/actions/runs/13475482660/job/37654246364#step:3:99)' (ID: 2635487389) deleted
Artifact 'coverage-22413' (ID: 2635489105) deleted
The 9 artifact(s) have been deleted
Run set -eux
+ python -Iim coverage combine
Traceback (most recent call last):
  File "<frozen runpy>", line 1[98](https://github.com/ipython/ipykernel/actions/runs/13475482660/job/37654246364#step:3:103), in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/coverage/__main__.py", line 10, in <module>
    sys.exit(main())
    ~~~~~~~~^^^^^^^^
SystemExit: 1
>>> 
No data to combine
+ python -Iim coverage html --skip-covered --skip-empty
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/coverage/__main__.py", line 10, in <module>
    sys.exit(main())
    ~~~~~~~~^^^^^^^^
SystemExit: 1
>>> 
No data to report.
+ python -Iim coverage report
+ sed 's/^/    /'
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/coverage/__main__.py", line 10, in <module>
    sys.exit(main())
    ~~~~~~~~^^^^^^^^
SystemExit: 1
>>> 
+ python -Iim coverage report --fail-under=80
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/coverage/__main__.py", line 10, in <module>
    sys.exit(main())
    ~~~~~~~~^^^^^^^^
SystemExit: 1
>>> 
No data to report.