fix(api): allow PodGroup minMember of 0 (v0.14 backport)

[gshaibi]

Description

Backport of #1989 to v0.14.

Adapted for this branch: MinMember here is a plain int32 (not *int32 as on main) and minSubGroup does not exist, so:

• CRD/kubebuilder marker relaxed from Minimum=1 to Minimum=0: explicit minMember: 0 (e.g. knative scale-to-zero podgroups) is no longer rejected by the API server.
• The scheduler's existing default of minAvailable 1 is kept: since a non-pointer int32 with omitempty cannot distinguish unset from explicit 0, honoring 0 would silently flip gang semantics for PodGroups created without minMember (this is asserted by TestSnapshotPodGroups). The full elastic minMember=0 behavior from main is therefore not backported.
• The minSubGroup webhook/e2e changes from the original PR are dropped (field does not exist on this branch).

Same adaptation as #2086 (v0.12).

Related Issues

Backport of #1989.

Checklist

• Self-reviewed
• Added/updated tests (if needed)
• Updated documentation (if needed)
• Added a changelog fragment via make changelog (or applied the skip-changelog label). Do not edit CHANGELOG.md directly — pending fragments are folded into it at release time.

Breaking Changes

None.

Additional Notes

Unit tests pass locally for pkg/scheduler/api/podgroup_info, pkg/scheduler/cache/..., pkg/podgrouper/.../knative, and pkg/apis/scheduling/v2alpha2. The v0.9 backport (#2087) was closed: that branch never had the Minimum=1 marker, so the bug does not exist there.

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[KaiPilotBot]

📝 Changelog fragment recorded

Thanks! This PR added the changelog fragment(s) below. Pending fragments are folded into CHANGELOG.md at release time, so it was intentionally not modified by this PR — your entry will appear in the next release:

• .changes/unreleased/fixed-20260728-212512.yaml