[V1][Spec Decode] Do not generate draft tokens beyond max_model_len

[WoosukKwon]

Implements 4. Handle the edge cases like when the draft model generates beyond max_pos_embeddings in #15901

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[comaniac]

Overall LGTM. Approve first to unblock the PR. Meanwhile it would be good to have a unit test for it. Also do we know the overhead of introduced ops (e.g., torch.where)?