feat(libcgroups): Support sub-cgroups for unified systemd cgroup manager

[logica0419]

Description

Adds support for sub-cgroups in the unified systemd cgroup manager.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test updates
• CI/CD related changes
• Other (please describe):

Testing

• Added new unit tests
• Added new integration tests
• Ran existing test suite
• Tested manually (please provide steps)

Related Issues

fixes #3565
refs #3342
refs #3347
dependent #3564

Additional Context

#3347 (comment)

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for sub-cgroup paths within systemd-managed cgroups by extracting a sub-cgroup suffix from the unit name and propagating it through cgroup path construction and process addition.

Changes:

• New extract_sub_cgroup helper that splits a name like {id}/sub/init into base name and sub-cgroup suffix.
• construct_cgroups_path and add_task updated to incorporate the sub-cgroup path.
• New unit tests for sub-cgroup extraction and path construction.

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[logica0419]

Although I worked on this as part of #3564, I’ve opened a separate issue for this PR since the enhancement is necessary regardless of the approach we take.