fix: Ignoring CPU realtime on cgroupsv2 if set to zero

[CheatCodeSam]

Description

Requested to break up #3174 into two seperate issues by @YJDoc2.

This fixes an auxiliary issue of #2994, where nerdctl will set both linux.resources.cpu.realtimeRuntime and linux.resources.cpu.realtimePeriod to zero in the config.json despite youki trying to use cgroupsv2. This would cause a V2CpuControllerError to be thrown when the container is being created.

My solution is just to ignore them if using cgroupsv2 and they are both set to zero.

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)

Once the initial runtime spec was fixed, this was able to run with no problems on nerdctl.

Related Issues

Fixes part of #2994.

Additional Context

[Copilot]

Pull Request Overview

This PR updates both the cgroupsv2 and systemd CPU controllers to skip the realtime runtime/period error when both values are explicitly set to zero, avoiding a V2CpuControllerError or SystemdCpuError in that case.

• Adds a guard in apply to check for (Some(0), Some(0)) and bypass the realtime error.
• Mirrors the same change in the systemd CPU controller.
• Does not include new tests for the added branch.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/libcgroups/src/v2/cpu.rs	Skip realtime error when both runtime and period are zero
crates/libcgroups/src/systemd/cpu.rs	Skip realtime error when both runtime and period are zero (systemd)

Comments suppressed due to low confidence (3)

crates/libcgroups/src/v2/cpu.rs:89

• Add a unit or integration test covering the case where both realtime_runtime and realtime_period are zero to ensure this new branch is exercised.

let realtime_runtime = cpu.realtime_runtime();

crates/libcgroups/src/systemd/cpu.rs:45

• Add a test in the systemd CPU controller for the scenario where both realtime values are zero so this branch is verified.

let realtime_runtime = cpu.realtime_runtime();

crates/libcgroups/src/v2/cpu.rs:86

• The realtime-ignore logic is duplicated in both v2 and systemd modules; consider extracting it into a shared helper function to reduce code duplication.

impl Cpu {

[utam0k]

Please add the unit test for your change 🙏

[CheatCodeSam]

Tests have been added

[utam0k]

Thanks!