Guarantees the last partial delta chunk is emitted when the SSE stream closes (EOF, break, or `[DONE]` sentinel).
* Buffer `last_delta_data` and track `delta_count`
* Flush automatically once `delta_count >= chunk_size`
* Perform a final flush after the iterator ends
Signed-off-by: Sihyeon Jang <sihyeon.jang@navercorp.com>
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