Support Elastic Jobs via WorkloadSlices

[ichekrygin]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR introduces the foundational implementation of WorkloadSlices in Kueue, as proposed in KEP-77. WorkloadSlices enable controlled scaling of admitted workloads (e.g., scale-up) while preserving Kueue's scheduling guarantees and resource tracking semantics.

📌 Summary

• Introduces WorkloadSlice concept as a transient workload object representing a logical scale-up request.
• Enables mutable workload behavior using a dual-Workload model:
  • Original admitted workload.
  • A new WorkloadSlice with additional requested capacity (1:2 state during transition).
• Admission of the new WorkloadSlice triggers preemption of the original Workload, even if additional capacity is available, to enforce consistent admission-state transitions.
• Uses Pod scheduling gates (instead of spec.suspend) for gating new pods until the slice is admitted.
• Defaulting logic enables the feature automatically for supported jobs (e.g., batchv1.Job, RayJob).
• Ensures all new pods created during the transition are gated until the corresponding Workload is admitted.
• Aggregates admission state and lifecycle management into the core Workload controller flow.

📎 Additional Notes

• Fully backward-compatible with existing single-Workload flow.
• Includes tests for:
  • Slice creation logic.
  • Admission/preemption interaction.
  • Scheduling gate behavior.
• Documentation and KEP link updates to follow in separate PR.

⚠️ Known Limitations

• Multi-cluster support for WorkloadSlices is still a work in progress and will be addressed in this or follow-up PRs.

Which issue(s) this PR fixes:

Fixes #5528

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Support for Elastic (Dynamically Sized Jobs) in Alpha as designed in [KEP-77](https://github.com/kubernetes-sigs/kueue/tree/main/keps/77-dynamically-sized-jobs). 
The implementation supports resizing (scale up and down) of batch/v1.Job and is behind the Alpha 
`ElasticJobsViaWorkloadSlices` feature gate. Jobs which are subject to resizing need to have the
`kueue.x-k8s.io/elastic-job` annotation added at creation time.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ichekrygin / name: Illya Chekrygin (4e08884, 143d5c5, f63aa55, fdf7717, d9255a2, fc49d7a, dd4ce88, 99e5423, f2869e4, d8e115f, 9bbccd3, 0313da5, cabcd8e, dafd141, 4494d78, fe51aed, df97d77, de5aaa5, ebb7b69, c0e72ef, c60cfdb, af2da4d, 8707058, 1184741, 75c4f50, 0241eb7, 7bf19d6, 9f1096a, 76e5216, f1d4409, 0509da1, 5ecbc02, d8164cc, 29e607c, 922a8a0, 3afbbf4, 083e71c, dcf4935, a29432a, af552b3, aa2aefd)

[k8s-ci-robot]

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[mimowo]

@ichekrygin I think this is very close to be mergable. I left a bunch of comments, mostly renames to use "replacing" terminology consistently, rather than preemptions, because the mechanism only marginally relies on preemptions.

it would also be great to add integration tests for the happy path. The release is on Friday, so we still have a bit of time to address the comments I think.

Feel free to also squash the commits. There are 33 of them, I highly doubt anyone would like to be traversing them :)

[mimowo]

LGTM, but please address the remaining comments

[mimowo]

Let's make the note a bit more user-oriented, I think the workload-slices replacement is more of a technical detail. Putting a link to KEP77 is probably enough for interested readers.
/release-note-edit

Support for Elastic (Dynamically Sized Jobs) in Alpha as designed in [KEP-77](https://github.com/kubernetes-sigs/kueue/tree/main/keps/77-dynamically-sized-jobs). 
The implementation supports resizing (scale up and down) of batch/v1.Job and is behind the Alpha 
`ElasticJobsViaWorkloadSlices` feature gate. Jobs which are subject to resizing need to have the
`kueue.x-k8s.io/elastic-job` annotation added at creation time.

[mimowo]

/lgtm
/approev
Thank you for your relentless work on KEP-77 and this implementation PR. This is one of the oldest and most anticipated KEPs in Kueue. While we still have a long way to go (e.g., support for other Job CRDs, MultiKueue, TAS), this is a huge milestone, and I'm very happy to get this in.

FYI @tenzen-y: Since the release is approaching and all of my comments have been addressed, I am merging this now to avoid potential conflicts with other PRs. I've taken extra care to ensure all new code is behind the alpha feature gate. Please feel free to add any further comments or open a new issue for follow-up items. I'm confident we can address them.

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: b632836c18466b71df5bac3e1328c769844678ef

[mimowo]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ichekrygin, mimowo

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