fix(api-server): persist incomplete snapshot on asyncio.CancelledError too

Extends PR #15171 to also cover the server-side cancellation path (aiohttp
shutdown, request-level timeout) — previously only ConnectionResetError
triggered the incomplete-snapshot write, so cancellations left the store
stuck at the in_progress snapshot written on response.created.

Factors the incomplete-snapshot build into a _persist_incomplete_if_needed()
helper called from both the ConnectionResetError and CancelledError
branches; the CancelledError handler re-raises so cooperative cancellation
semantics are preserved.

Adds two regression tests that drive _write_sse_responses directly (the
TestClient disconnect path races the server handler, which makes the
end-to-end assertion flaky).

tests/gateway/test_api_server.py

@@ -1374,6 +1374,139 @@ class TestResponsesStreaming:
                 assert data["status"] == "completed"
                 assert data["output"][-1]["content"][0]["text"] == "Stored response"
 
+    @pytest.mark.asyncio
+    async def test_stream_cancelled_persists_incomplete_snapshot(self, adapter):
+        """Server-side asyncio.CancelledError (shutdown, request timeout) must
+        still leave an ``incomplete`` snapshot in ResponseStore so
+        GET /v1/responses/{id} and previous_response_id chaining keep
+        working.  Regression for PR #15171 follow-up.
+
+        Calls _write_sse_responses directly so the test can await the
+        handler to completion (TestClient disconnection races the server
+        handler, which makes end-to-end assertion on the final stored
+        snapshot flaky).
+        """
+        # Build a minimal fake request + stream queue the writer understands.
+        fake_request = MagicMock()
+        fake_request.headers = {}
+
+        written_payloads: list = []
+
+        class _FakeStreamResponse:
+            async def prepare(self, req):
+                pass
+
+            async def write(self, payload):
+                written_payloads.append(payload)
+
+        # Patch web.StreamResponse for the duration of the writer call.
+        import gateway.platforms.api_server as api_mod
+        import queue as _q
+
+        stream_q: _q.Queue = _q.Queue()
+
+        async def _agent_coro():
+            # Feed one partial delta into the stream queue...
+            stream_q.put("partial output")
+            # ...then give the drain loop a moment to pick it up before
+            # raising CancelledError to simulate a server-side cancel.
+            await asyncio.sleep(0.01)
+            raise asyncio.CancelledError()
+
+        agent_task = asyncio.ensure_future(_agent_coro())
+        response_id = f"resp_{uuid.uuid4().hex[:28]}"
+
+        with patch.object(api_mod.web, "StreamResponse", return_value=_FakeStreamResponse()):
+            with pytest.raises(asyncio.CancelledError):
+                await adapter._write_sse_responses(
+                    request=fake_request,
+                    response_id=response_id,
+                    model="hermes-agent",
+                    created_at=int(time.time()),
+                    stream_q=stream_q,
+                    agent_task=agent_task,
+                    agent_ref=[None],
+                    conversation_history=[],
+                    user_message="will be cancelled",
+                    instructions=None,
+                    conversation=None,
+                    store=True,
+                    session_id=None,
+                )
+
+        # The in_progress snapshot was persisted on response.created,
+        # and the CancelledError handler must have updated it to
+        # ``incomplete`` with the partial text it saw.
+        stored = adapter._response_store.get(response_id)
+        assert stored is not None, "snapshot must be retrievable after cancellation"
+        assert stored["response"]["status"] == "incomplete"
+        # Partial text captured before cancel should be preserved.
+        output_text = "".join(
+            part.get("text", "")
+            for item in stored["response"].get("output", [])
+            if item.get("type") == "message"
+            for part in item.get("content", [])
+        )
+        assert "partial output" in output_text
+
+    @pytest.mark.asyncio
+    async def test_stream_client_disconnect_persists_incomplete_snapshot(self, adapter):
+        """Client disconnect (ConnectionResetError) during streaming must
+        persist an ``incomplete`` snapshot in ResponseStore.  Regression
+        for PR #15171."""
+        fake_request = MagicMock()
+        fake_request.headers = {}
+
+        write_call_count = {"n": 0}
+
+        class _DisconnectingStreamResponse:
+            async def prepare(self, req):
+                pass
+
+            async def write(self, payload):
+                # First two writes succeed (prepare + response.created).
+                # On the third write (a text delta), the "client"
+                # disconnects — simulate with ConnectionResetError.
+                write_call_count["n"] += 1
+                if write_call_count["n"] >= 3:
+                    raise ConnectionResetError("simulated client disconnect")
+
+        import gateway.platforms.api_server as api_mod
+        import queue as _q
+
+        stream_q: _q.Queue = _q.Queue()
+        stream_q.put("some streamed text")
+        stream_q.put(None)  # EOS sentinel
+
+        async def _agent_coro():
+            await asyncio.sleep(0.01)
+            return ({"final_response": "", "messages": [], "api_calls": 0},
+                    {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0})
+
+        agent_task = asyncio.ensure_future(_agent_coro())
+        response_id = f"resp_{uuid.uuid4().hex[:28]}"
+
+        with patch.object(api_mod.web, "StreamResponse", return_value=_DisconnectingStreamResponse()):
+            await adapter._write_sse_responses(
+                request=fake_request,
+                response_id=response_id,
+                model="hermes-agent",
+                created_at=int(time.time()),
+                stream_q=stream_q,
+                agent_task=agent_task,
+                agent_ref=[None],
+                conversation_history=[],
+                user_message="will disconnect",
+                instructions=None,
+                conversation=None,
+                store=True,
+                session_id=None,
+            )
+
+        stored = adapter._response_store.get(response_id)
+        assert stored is not None, "snapshot must survive client disconnect"
+        assert stored["response"]["status"] == "incomplete"
+
 
 # ---------------------------------------------------------------------------
 # Auth on endpoints