[BugFix] Fix handling of num_computed_tokens with connector

[njhill]

For async loading there are kind of two passes int the scheduler for the request. The first time, the connector methods are called and it goes into WAITING_FOR_REMOTE_KVS state.

Once the async load is ready, it goes through the "waiting" part of the schedule() method again for that request... previously it was calling the connector methods again when this happened, even though we don't want to do any more since the computation is now done.

This originally wasn't causing a problem because the nixl connector unsets the flag in the transfer params to that it's ignored the second tim.

But in the multi-connector, when get_num_matched_tokens returns 0 from one of the connectors it moves on to the next one ... so in this second pass we were actually triggering the LMCache connector.

So this change makes things more explicit/robust in the scheduler w.r.t. when the connectors are invoked. Similarly it also only calls update_state_after_alloc if get_num_matched_tokens returned nonzero.. since in terms of the API contract it doesn't really make sense to invoke the connector after the allocation if it's saying that it is not providing any tokens.

Same as upstream PR vllm-project#18232

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