feat: emit empty completion chunk along with keep-alive heartbeat in chat stream

internal/httpapi/openai/chat/chat_stream_runtime.go

@@ -122,7 +122,13 @@ func (s *chatStreamRuntime) sendKeepAlive() {
 		return
 	}
 	_, _ = s.w.Write([]byte(": keep-alive\n\n"))
-	_ = s.rc.Flush()
+	s.sendChunk(openaifmt.BuildChatStreamChunk(
+		s.completionID,
+		s.created,
+		s.model,
+		[]map[string]any{},
+		nil,
+	))
 }
 
 func (s *chatStreamRuntime) sendChunk(v any) {

internal/httpapi/openai/chat/chat_stream_runtime_test.go

@@ -0,0 +1,53 @@
+package chat
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestChatStreamKeepAliveEmitsEmptyChoiceDataFrame(t *testing.T) {
+	rec := httptest.NewRecorder()
+	runtime := newChatStreamRuntime(
+		rec,
+		http.NewResponseController(rec),
+		true,
+		"chatcmpl-test",
+		time.Now().Unix(),
+		"deepseek-v4-flash",
+		"prompt",
+		false,
+		false,
+		true,
+		nil,
+		nil,
+		false,
+		false,
+	)
+
+	runtime.sendKeepAlive()
+
+	body := rec.Body.String()
+	if !strings.Contains(body, ": keep-alive\n\n") {
+		t.Fatalf("expected keep-alive comment, got %q", body)
+	}
+	frames, done := parseSSEDataFrames(t, body)
+	if done {
+		t.Fatalf("keep-alive must not emit [DONE], body=%q", body)
+	}
+	if len(frames) != 1 {
+		t.Fatalf("expected one data frame, got %d body=%q", len(frames), body)
+	}
+	if got := asString(frames[0]["id"]); got != "chatcmpl-test" {
+		t.Fatalf("expected completion id to be preserved, got %q", got)
+	}
+	if got := asString(frames[0]["object"]); got != "chat.completion.chunk" {
+		t.Fatalf("expected chat chunk object, got %q", got)
+	}
+	choices, _ := frames[0]["choices"].([]any)
+	if len(choices) != 0 {
+		t.Fatalf("expected empty choices heartbeat, got %#v", choices)
+	}
+}