[release-1.14] fix: fall back minAvailable to replicas when partition minPartitions is omitted

[Copilot]

Manual cherry-pick of #5402 to release-1.14 (automated cherry-pick failed due to test struct differences).

When PartitionPolicy is set but MinPartitions is omitted (defaults to 0), the mutate webhook computed MinAvailable = 0 * PartitionSize = 0, silently dropping the task's gang scheduling guarantee. The validate webhook then rejected the corrected value because 0 != MinAvailable.

Changes

• mutate_job.go — Gate partition-derived MinAvailable on MinPartitions > 0; otherwise fall back to Replicas
• admit_job.go — Skip MinAvailable == MinPartitions*PartitionSize validation when MinPartitions is 0
• mutate_job_test.go — Fix existing expectation (MinAvailable = 1 not 0) and add regression test
• admit_job_test.go — Add regression test (adapted to release-1.14 test struct which lacks PodLevelResourcesEnabled)

// Before: MinPartitions=0 yields MinAvailable=0
if tasks[index].PartitionPolicy != nil {
    minAvailable := tasks[index].PartitionPolicy.MinPartitions * tasks[index].PartitionPolicy.PartitionSize

// After: only use partition math when MinPartitions is explicitly set
if tasks[index].PartitionPolicy != nil && tasks[index].PartitionPolicy.MinPartitions > 0 {
    minAvailable := tasks[index].PartitionPolicy.MinPartitions * tasks[index].PartitionPolicy.PartitionSize

[JesseStutler]

/approve
/lgtm

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