core/lmproto: stop SSE stream at data: [DONE]

Previously DONE was skipped with continue, so keep-alive connections
could hang forever after the model finished. Also fail fast on HTTP
error statuses for stream and non-stream completions.

tests/test_lmproto/test_sse_parse.py

@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, cast
+
+from plyngent.lmproto.openai_compatible.client import BaseOpenAIClient
+from plyngent.lmproto.openai_compatible.config import OpenAIConfig
+
+if TYPE_CHECKING:
+    from collections.abc import AsyncIterator
+
+    from niquests.models import AsyncResponse
+
+
+class _FakeResp:
+    status_code: int
+    closed: bool
+    content: bytes
+    _lines: list[bytes]
+
+    def __init__(self, lines: list[bytes], *, status_code: int = 200) -> None:
+        self.status_code = status_code
+        self.closed = False
+        self._lines = lines
+        self.content = b""
+
+    async def iter_lines(self) -> AsyncIterator[bytes]:
+        for line in self._lines:
+            yield line
+
+    def close(self) -> None:
+        self.closed = True
+
+
+async def test_parse_sse_stops_at_done() -> None:
+    client = BaseOpenAIClient(OpenAIConfig(access_key_or_token="t", base_url="http://x"))
+    # Minimal OpenAI-style chunk payload
+    chunk = (
+        b'data: {"id":"1","object":"chat.completion.chunk","created":0,'
+        b'"model":"m","choices":[{"index":0,"delta":{"content":"hi"},'
+        b'"finish_reason":null}]}'
+    )
+    resp = _FakeResp([chunk, b"", b"data: [DONE]", b"data: should-not-parse"])
+    out = [c async for c in client._parse_sse(cast("AsyncResponse", cast("object", resp)))]
+    assert len(out) == 1
+    assert out[0].choices[0].delta.content == "hi"
+    assert resp.closed is True
+
+
+async def test_parse_sse_http_error() -> None:
+    import pytest
+
+    client = BaseOpenAIClient(OpenAIConfig(access_key_or_token="t", base_url="http://x"))
+    resp = _FakeResp([], status_code=500)
+    resp.content = b'{"error":"boom"}'
+    with pytest.raises(RuntimeError, match="500"):
+        _ = [c async for c in client._parse_sse(cast("AsyncResponse", cast("object", resp)))]
+    assert resp.closed is True