fix: remove OpenAI unknown model fallback

backend/internal/service/openai_gateway_chat_completions_test.go

@@ -97,6 +97,42 @@ func TestNormalizeResponsesBodyServiceTier(t *testing.T) {
 	require.False(t, gjson.GetBytes(body, "service_tier").Exists())
 }
 
+func TestForwardAsChatCompletions_UnknownModelDoesNotUseDefaultMappedModel(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	body := []byte(`{"model":"gpt6","messages":[{"role":"user","content":"hello"}],"stream":false}`)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	upstream := &httpUpstreamRecorder{resp: &http.Response{
+		StatusCode: http.StatusBadRequest,
+		Header:     http.Header{"Content-Type": []string{"application/json"}, "x-request-id": []string{"rid_chat_unknown_model"}},
+		Body:       io.NopCloser(strings.NewReader(`{"error":{"type":"invalid_request_error","message":"model not found"}}`)),
+	}}
+
+	svc := &OpenAIGatewayService{httpUpstream: upstream}
+	account := &Account{
+		ID:          1,
+		Name:        "openai-oauth",
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeOAuth,
+		Concurrency: 1,
+		Credentials: map[string]any{
+			"access_token":       "oauth-token",
+			"chatgpt_account_id": "chatgpt-acc",
+		},
+	}
+
+	result, err := svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "gpt-5.4")
+	require.Error(t, err)
+	require.Nil(t, result)
+	require.Equal(t, "gpt6", gjson.GetBytes(upstream.lastBody, "model").String())
+	require.NotEqual(t, "gpt-5.4", gjson.GetBytes(upstream.lastBody, "model").String())
+	require.Equal(t, http.StatusBadRequest, rec.Code)
+}
+
 func TestForwardAsChatCompletions_ClientDisconnectDrainsUpstreamUsage(t *testing.T) {
 	gin.SetMode(gin.TestMode)