Partial Admission Preemption Panic

[gabesaba]

What happened:
While refactoring Preemptor.GetTargets, I encountered a panic. Furthermore, I discovered that we are not exercising the PartialAdmission preemption logic in any of our tests, and it may be broken and panicking generally.

This test exits here, as we find a preemption target above. The PartialAdmission preemption path is not tested.

In this test, we exit here. We panic when calling assignment.TotalRequestsFor, as this map is empty

How to reproduce it (as minimally and precisely as possible):

Move this line to the top of Preemptor.GetTargets and observe a panic during this test

kueue/pkg/scheduler/preemption/preemption.go

Line 116 in 00111d9

wlReq := assignment.TotalRequestsFor(&wl)

Delete these lines and observe that unit and integration tests still pass

kueue/pkg/scheduler/scheduler.go

Lines 477 to 481 in 00111d9

	preemptionTargets := s.preemptor.GetTargets(log, *wl, assignment, snap)
	if len(preemptionTargets) > 0 {
	return &partialAssignment{assignment: assignment, preemptionTargets: preemptionTargets}, true
	}
	return nil, false

[gabesaba]

cc @mimowo @mbobrovskyi

[mbobrovskyi]

/assign @trasc