test(client): pin streaming terminator finish_reason parity

New tests/test_handle_chat_stream_finish.py drives _handle_chat directly
with a mock ws for both async-gen and sync-gen handlers that yield a
finish_reason="length" chunk, asserting the terminator carries "length"
not "stop"; a third case pins the pre-existing "stop" default when the
handler stays silent so the fallback doesn't regress. Mutation-verified
against the pre-fix client.py (2 fail with "stop" on the wire).

The existing tests/test_client_sync_generator.py had a line pinning the
terminator to "stop" while the handler yielded finish_reason="length" —
that assertion was pinning the very bug being fixed. Updated to expect
"length" on the terminator, matching the JS port's semantics.

Author: Zawwarsami16

tests/test_client_sync_generator.py

@@ -51,8 +51,10 @@ def handler(messages, options):
     # The chunk's own done/finish_reason survive serialization.
     assert payloads[1]["done"] is True
     assert payloads[1]["finish_reason"] == "length"
-    # Trailing synthetic terminator closes the stream.
-    assert payloads[-1] == {"delta": "", "done": True, "finish_reason": "stop"}
+    # Trailing synthetic terminator closes the stream and echoes the
+    # handler-supplied finish_reason (here 'length' — max-tokens truncation),
+    # not a hardcoded 'stop' that would misrepresent the outcome.
+    assert payloads[-1] == {"delta": "", "done": True, "finish_reason": "length"}
 
 
 @pytest.mark.asyncio

tests/test_handle_chat_stream_finish.py

@@ -0,0 +1,101 @@
+"""Regression: _handle_chat's streaming paths (async-gen + sync-gen) hardcoded
+`finish_reason="stop"` on the terminator chat-chunk regardless of what the
+handler yielded. A handler that flagged a max-tokens truncation with
+`finish_reason="length"` had it silently rewritten to `"stop"` on the wire,
+so the client couldn't distinguish clean stop from truncation.
+
+The non-streaming accumulate paths already carry the handler's finish_reason
+through `_finalize_accumulated`. The JS port was fixed for the streaming
+paths in 7427ed4 — this brings Python back to parity.
+"""
+
+import json
+
+import pytest
+
+from zhub.client import _handle_chat
+from zhub.protocol import Envelope
+
+
+class _FakeWS:
+    def __init__(self) -> None:
+        self.sent: list[dict] = []
+
+    async def send(self, text: str) -> None:
+        self.sent.append(json.loads(text))
+
+
+class _Pub:
+    def __init__(self, handler) -> None:
+        self.chat_handler = handler
+
+
+def _last_chunk(ws: _FakeWS) -> dict:
+    """The terminator envelope — the last chat-chunk on the wire."""
+    chunks = [e for e in ws.sent if e["type"] == "chat-chunk"]
+    assert chunks, f"no chat-chunk envelopes emitted; got {[e['type'] for e in ws.sent]}"
+    return chunks[-1]
+
+
+@pytest.mark.asyncio
+async def test_asyncgen_streaming_terminator_echoes_handler_finish_reason():
+    """Handler yields a final chunk with finish_reason='length'. The terminator
+    chat-chunk (done=True) must carry finish_reason='length', not 'stop'."""
+
+    async def handler(messages, options):
+        yield "hello "
+        yield "world"
+        yield {"delta": "", "finish_reason": "length"}
+
+    ws = _FakeWS()
+    env = Envelope(
+        type="chat-request",
+        payload={"messages": [], "stream": True},
+    )
+    await _handle_chat(_Pub(handler), ws, env)
+
+    terminator = _last_chunk(ws)
+    assert terminator["payload"]["done"] is True
+    assert terminator["payload"]["finish_reason"] == "length"
+
+
+@pytest.mark.asyncio
+async def test_syncgen_streaming_terminator_echoes_handler_finish_reason():
+    """Same guarantee for a sync generator handler."""
+
+    def handler(messages, options):
+        yield "partial "
+        yield "answer"
+        yield {"delta": "", "finish_reason": "length"}
+
+    ws = _FakeWS()
+    env = Envelope(
+        type="chat-request",
+        payload={"messages": [], "stream": True},
+    )
+    await _handle_chat(_Pub(handler), ws, env)
+
+    terminator = _last_chunk(ws)
+    assert terminator["payload"]["done"] is True
+    assert terminator["payload"]["finish_reason"] == "length"
+
+
+@pytest.mark.asyncio
+async def test_asyncgen_streaming_defaults_to_stop_when_handler_silent():
+    """Handler never sets finish_reason → terminator carries the default 'stop'.
+    Regression guard so the fix doesn't drop the pre-existing default."""
+
+    async def handler(messages, options):
+        yield "one"
+        yield "two"
+
+    ws = _FakeWS()
+    env = Envelope(
+        type="chat-request",
+        payload={"messages": [], "stream": True},
+    )
+    await _handle_chat(_Pub(handler), ws, env)
+
+    terminator = _last_chunk(ws)
+    assert terminator["payload"]["done"] is True
+    assert terminator["payload"]["finish_reason"] == "stop"