[Core] Ignore infeasible swap requests.

[rkooo567]

Currently, when a request is swapped out, and if it is infeasible because we don't have enough kv caches to schedule the sequence, it hangs forever. We already handle this case correctly for preemption case using AllocStatus.NEVER. We are using the same mechanism to ignore swapped out requests that cannot be scheduled.

This also adds test cases for infeasible preemption requests.

TODO

• Add scheduler/block manager unit tests

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[rkooo567]

not used

[robertgshaw2-neuralmagic]

Thanks @rkooo567 @simon-mo