fix(api): allow PodGroup minMember of 0 (v0.9 backport)

[gshaibi]

Description

Backport of #1989 to v0.9.

Adapted for this branch: MinMember here is a plain int32 (not *int32 as on main), there is no minSubGroup, and this branch never had a Minimum=1 CRD marker (explicit minMember: 0 was already accepted by the API but silently clamped to 1 by the scheduler). Changes:

• Scheduler no longer clamps minMember to 1 (max(minMember, 0) instead of max(minMember, 1)), so minMember 0 now means no gang requirement (e.g. knative scale-to-zero).
• Doc comment added to the API type; CRD description regenerated via make manifests.
• The minSubGroup webhook/e2e changes from the original PR are dropped (field does not exist on this branch).

Note: since int32 with omitempty cannot distinguish an unset minMember from an explicit 0, a PodGroup created without minMember now gets minAvailable 0 (elastic) instead of the previous implicit 1. Podgrouper-created PodGroups always set it, so this only affects hand-written PodGroups.

Same adaptation as the v0.14 (#2085) and v0.12 (#2086) backports.

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 for pkg/scheduler/api/podgroup_info and pkg/podgrouper/.../knative.

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[gshaibi]

Closing: the bug does not exist on v0.9. This branch never had the Minimum=1 CRD marker (nor a webhook check), so minMember: 0 is already accepted by the API. The scheduler's default of minAvailable 1 when minMember is 0/unset is the existing branch contract (MinMember is a non-pointer int32 here, so unset and explicit 0 are indistinguishable), and changing it would alter gang semantics for existing PodGroups in a patch release — see the v0.14/v0.12 backports #2085/#2086 which keep that behavior for the same reason.