fix: check partial stop on cumulative sglang output

[Chessing234]

Summary

SGLang streaming dropped newlines when a stop string started with \n (e.g. \n<|endoftext|>).

Root cause

sglang_worker.py called is_partial_stop(out, stop) on each delta chunk. A lone \n chunk matched as a prefix of \n<|endoftext|>, so the worker continued without ever appending it to entire_output — the newline was discarded permanently.

Fix

Append each chunk first, then run the partial-stop check on the cumulative entire_output (same pattern as vllm_worker). If the suffix is only a tentative stop prefix, skip yielding for that step but keep the text buffered until the next token confirms it is not the stop sequence.

Fixes #3467

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