fix: remove OpenAI unknown model fallback

backend/internal/service/openai_oauth_passthrough_test.go

@@ -48,6 +48,49 @@ func (u *httpUpstreamRecorder) DoWithTLS(req *http.Request, proxyURL string, acc
 	return u.Do(req, proxyURL, accountID, accountConcurrency)
 }
 
+func TestOpenAIGatewayService_ResponsesUnknownModelDoesNotFallbackToGPT54(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	originalBody := []byte(`{"model":"gpt6","stream":false,"instructions":"local-test-instructions","input":[{"type":"text","text":"hi"}]}`)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", bytes.NewReader(originalBody))
+	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_unknown_model"}},
+		Body:       io.NopCloser(strings.NewReader(`{"error":{"type":"invalid_request_error","message":"model not found"}}`)),
+	}}
+
+	svc := &OpenAIGatewayService{
+		cfg:          &config.Config{},
+		httpUpstream: upstream,
+	}
+	account := &Account{
+		ID:          123,
+		Name:        "acc",
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeOAuth,
+		Concurrency: 1,
+		Credentials: map[string]any{
+			"access_token":       "oauth-token",
+			"chatgpt_account_id": "chatgpt-acc",
+		},
+		Status:      StatusActive,
+		Schedulable: true,
+	}
+
+	result, err := svc.Forward(context.Background(), c, account, originalBody)
+	require.Error(t, err)
+	require.Nil(t, result)
+	require.NotNil(t, upstream.lastReq)
+	require.Equal(t, "https://chatgpt.com/backend-api/codex/responses", upstream.lastReq.URL.String())
+	require.Equal(t, "gpt6", gjson.GetBytes(upstream.lastBody, "model").String())
+	require.NotEqual(t, "gpt-5.4", gjson.GetBytes(upstream.lastBody, "model").String())
+	require.True(t, rec.Code >= http.StatusBadRequest)
+}
+
 type openAIPassthroughFailoverRepo struct {
 	stubOpenAIAccountRepo
 	rateLimitCalls []time.Time