test(openai): isolate visible TTFT fixture

backend/internal/service/openai_first_output_timeout_test.go

@@ -403,6 +403,16 @@ func assertOpenAINativeLargeOpenEventTimesOutWithoutLeak(t *testing.T, line stri
 }
 
 func TestOpenAINativeFirstOutputEOFDispatchesTerminalEventWithoutBlankLine(t *testing.T) {
+	gatewayForwardingCache.Store(&cachedGatewayForwardingSettings{
+		openAITTFTMode: OpenAITTFTModeVisible,
+		expiresAt:      time.Now().Add(time.Minute).UnixNano(),
+	})
+	t.Cleanup(func() {
+		gatewayForwardingCache.Store(&cachedGatewayForwardingSettings{
+			openAITTFTMode: OpenAITTFTModeSemantic,
+			expiresAt:      time.Now().Add(time.Minute).UnixNano(),
+		})
+	})
 	cfg := &config.Config{Gateway: config.GatewayConfig{
 		OpenAIFirstOutputTimeoutSeconds: 1,
 		MaxLineSize:                     defaultMaxLineSize,