Applies CCCL cub/device/dispatch/dispatch_rle.cuh design:
- streaming_context bundles all partition state (double-buffered prefix,
num_accumulated_uniques) into one struct passed through sweep kernel
- Translated: merge scattered parallel arrays (previous_num_tokens,
finish_reason_sent, reasoning_end_arr, reasoning_token_counts) into
a unified streaming context block per choice index
- Each choice is a 'partition' with isolated state, matching CCCL's
per-partition streaming_context<T> pattern
- Eliminates duplicate reasoning_end_arr/reasoning_token_counts declarations