fix: Ensure SSE streams flush trailing events (#6155)

## Summary
- ensure both async and sync HTTP clients flush the SSE decoder after
streaming
- add regression tests covering trailing SSE events without a
terminating blank line

## Testing
- make format
- make lint
- make test

------
https://chatgpt.com/codex/tasks/task_e_68c9727ca9f8832d9f207323c5e02a72

libs/sdk-py/langgraph_sdk/client.py

@@ -483,6 +483,8 @@ class HttpClient:
                 sse = decoder.decode(line=line.rstrip(b"\n"))
                 if sse is not None:
                     yield sse
+            if sse := decoder.decode(b""):
+                yield sse
 
 
 async def _aencode_json(json: Any) -> tuple[dict[str, str], bytes | None]:
@@ -3673,6 +3675,8 @@ class SyncHttpClient:
                 sse = decoder.decode(line.rstrip(b"\n"))
                 if sse is not None:
                     yield sse
+            if sse := decoder.decode(b""):
+                yield sse
 
 
 def _encode_json(json: Any) -> tuple[dict[str, str], bytes]:

libs/sdk-py/tests/test_client_stream.py

@@ -0,0 +1,81 @@
+from collections.abc import Iterator
+from pathlib import Path
+
+import httpx
+import pytest
+
+from langgraph_sdk.client import HttpClient, SyncHttpClient
+from langgraph_sdk.schema import StreamPart
+from langgraph_sdk.sse import BytesLike, BytesLineDecoder, SSEDecoder
+
+with open(Path(__file__).parent / "fixtures" / "response.txt", "rb") as f:
+    RESPONSE_PAYLOAD = f.read()
+
+
+def iter_lines_raw(payload: list[bytes]) -> Iterator[BytesLike]:
+    decoder = BytesLineDecoder()
+    for part in payload:
+        yield from decoder.decode(part)
+    yield from decoder.flush()
+
+
+def test_stream_see():
+    for groups in (
+        [RESPONSE_PAYLOAD],
+        RESPONSE_PAYLOAD.splitlines(keepends=True),
+    ):
+        parts: list[StreamPart] = []
+
+        decoder = SSEDecoder()
+        for line in iter_lines_raw(groups):
+            sse = decoder.decode(line=line.rstrip(b"\n"))
+            if sse is not None:
+                parts.append(sse)
+        if sse := decoder.decode(b""):
+            parts.append(sse)
+
+        assert decoder.decode(b"") is None
+        assert len(parts) == 79
+
+
+@pytest.mark.asyncio
+async def test_http_client_stream_flushes_trailing_event():
+    payload = b'event: foo\ndata: {"bar": 1}\n'
+
+    async def handler(request: httpx.Request) -> httpx.Response:
+        assert request.headers["accept"] == "text/event-stream"
+        assert request.headers["cache-control"] == "no-store"
+        return httpx.Response(
+            200,
+            headers={"Content-Type": "text/event-stream"},
+            content=payload,
+        )
+
+    transport = httpx.MockTransport(handler)
+    async with httpx.AsyncClient(
+        transport=transport, base_url="https://example.com"
+    ) as client:
+        http_client = HttpClient(client)
+        parts = [part async for part in http_client.stream("/stream", "GET")]
+
+    assert parts == [StreamPart(event="foo", data={"bar": 1})]
+
+
+def test_sync_http_client_stream_flushes_trailing_event():
+    payload = b'event: foo\ndata: {"bar": 1}\n'
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        assert request.headers["accept"] == "text/event-stream"
+        assert request.headers["cache-control"] == "no-store"
+        return httpx.Response(
+            200,
+            headers={"Content-Type": "text/event-stream"},
+            content=payload,
+        )
+
+    transport = httpx.MockTransport(handler)
+    with httpx.Client(transport=transport, base_url="https://example.com") as client:
+        http_client = SyncHttpClient(client)
+        parts = list(http_client.stream("/stream", "GET"))
+
+    assert parts == [StreamPart(event="foo", data={"bar": 1})]