fix: emit response.failed when compact keepalive commits headers but no SSE payload

When OpenAI Responses streaming has compact keepalive commit 200 headers
but no semantic SSE event has been written, upstream 4xx errors were
silently swallowed, causing clients to hang until timeout.

The fix distinguishes between 'headers committed' and 'meaningful SSE
output written'. Compact keepalive may commit headers for heartbeat
purposes without writing actual SSE events. In that case, the Responses
stream still needs its protocol-correct terminal response.failed event.

Preserves existing behavior when a complete upstream response or
meaningful SSE payload was already communicated, and preserves image
keepalive semantics.

Fixes #5289

backend/internal/handler/openai_gateway_handler.go

@@ -2696,7 +2696,11 @@ func (h *OpenAIGatewayHandler) ensureForwardErrorResponse(c *gin.Context, stream
 		imageKeepalivePaddingOnly = adjustedSize < 0
 		imageKeepaliveResponseWritten = adjustedSize >= 0
 	}
-	if service.IsResponseCommitted(c) || (!compactKeepaliveCommitted && imageKeepaliveResponseWritten) {
+	compactKeepaliveHasMeaningfulOutput := compactKeepaliveCommitted && service.OpenAICompactKeepaliveAdjustedWrittenSize(c) > 0
+	// Compact keepalive may have committed 200 headers without writing a
+	// semantic SSE event. In that case the Responses stream still needs its
+	// protocol-correct terminal response.failed event.
+	if (service.IsResponseCommitted(c) && (!compactKeepaliveCommitted || compactKeepaliveHasMeaningfulOutput)) || (!compactKeepaliveCommitted && imageKeepaliveResponseWritten) {
 		return false
 	}
 	if c.Writer.Written() && !imageKeepalivePaddingOnly {

backend/internal/handler/openai_gateway_handler_test.go

@@ -334,6 +334,26 @@ func TestOpenAIEnsureForwardErrorResponse_AfterDeltaAppendsSingleValidResponseFa
 	require.Equal(t, 1, errorEvents)
 }
 
+func TestOpenAIEnsureForwardErrorResponse_CompactKeepaliveOnlyWritesResponseFailed(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodPost, EndpointResponses, nil)
+	service.MarkOpenAICompactClientStream(c)
+
+	stop := service.StartOpenAICompactSSEKeepalive(c, 5*time.Millisecond)
+	defer stop()
+	before := service.OpenAICompactKeepaliveAdjustedWrittenSize(c)
+	require.Eventually(t, c.Writer.Written, time.Second, time.Millisecond)
+	require.Equal(t, before, service.OpenAICompactKeepaliveAdjustedWrittenSize(c))
+
+	h := &OpenAIGatewayHandler{}
+	require.True(t, h.ensureForwardErrorResponse(c, false))
+	require.Equal(t, http.StatusOK, w.Code)
+	require.Contains(t, w.Body.String(), "event: response.failed\n")
+	require.NotContains(t, w.Body.String(), "event: error\n")
+}
+
 func TestOpenAIEnsureForwardErrorResponse_ImageJSONKeepaliveWritesSingleJSONFallback(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	w := httptest.NewRecorder()