Restructure coverage handling with dedicated cover-linux job

[blink1073]

Summary

• test-linux and test-other-ubuntu no longer collect or upload coverage (run just test only)
• Add cover-linux job (ubuntu-latest, Python 3.10) as the dedicated Linux coverage runner; uploads directly to Codecov with flags: linux
• test-other-systems uploads coverage to Codecov only for windows-latest with flags: windows
• Add [tool.coverage.run] relative_files = true so coverage XML paths resolve correctly across platforms
• Set after_n_builds: 2 in codecov.yml to wait for both uploads (linux + windows) before notifying
• Add cover-linux to tests_check branch protection needs
• Add tests for PDF class and get_kernel_json with OCTAVE_KERNEL_JSON env var

Test plan

• Verify cover-linux runs and uploads coverage to Codecov with flags: linux
• Verify test-other-systems uploads coverage only for windows-latest with flags: windows
• Verify test-linux and test-other-ubuntu run tests without uploading coverage
• Verify Codecov notifies after both uploads complete
• Verify new TestPDF and TestGetKernelJson tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.54%. Comparing base (87c234d) to head (e0947fa).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #291   +/-   ##
=======================================
  Coverage   91.54%   91.54%           
=======================================
  Files           6        6           
  Lines         426      426           
=======================================
  Hits          390      390           
  Misses         36       36           

Flag	Coverage Δ
linux	91.31% <ø> (?)
linux-py3.10	?
linux-py3.11	?
linux-py3.12	?
linux-py3.13	?
linux-py3.14	?
linux-py3.14t	?
macos-latest	?
ubuntu-flatpak	?
ubuntu-snap	?
windows	91.54% <ø> (?)
windows-latest	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
octave_kernel/kernel.py	92.47% <ø> (ø)

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