feat(openai): 支持自动透传开关并透传 User-Agent

- OpenAI OAuth/API Key 统一支持自动透传开关，编辑页可开关\n- 透传模式仅替换认证并保留计费/并发/审计，修复 API Key responses 端点拼接\n- Usage 页面显示原始 User-Agent 且不截断，补充回归测试与清单

backend/internal/service/account_openai_passthrough_test.go

@@ -0,0 +1,72 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAccount_IsOpenAIPassthroughEnabled(t *testing.T) {
+	t.Run("新字段开启", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformOpenAI,
+			Type:     AccountTypeAPIKey,
+			Extra: map[string]any{
+				"openai_passthrough": true,
+			},
+		}
+		require.True(t, account.IsOpenAIPassthroughEnabled())
+	})
+
+	t.Run("兼容旧字段", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformOpenAI,
+			Type:     AccountTypeOAuth,
+			Extra: map[string]any{
+				"openai_oauth_passthrough": true,
+			},
+		}
+		require.True(t, account.IsOpenAIPassthroughEnabled())
+	})
+
+	t.Run("非OpenAI账号始终关闭", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformAnthropic,
+			Type:     AccountTypeOAuth,
+			Extra: map[string]any{
+				"openai_passthrough": true,
+			},
+		}
+		require.False(t, account.IsOpenAIPassthroughEnabled())
+	})
+
+	t.Run("空额外配置默认关闭", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformOpenAI,
+			Type:     AccountTypeOAuth,
+		}
+		require.False(t, account.IsOpenAIPassthroughEnabled())
+	})
+}
+
+func TestAccount_IsOpenAIOAuthPassthroughEnabled(t *testing.T) {
+	t.Run("仅OAuth类型允许返回开启", func(t *testing.T) {
+		oauthAccount := &Account{
+			Platform: PlatformOpenAI,
+			Type:     AccountTypeOAuth,
+			Extra: map[string]any{
+				"openai_passthrough": true,
+			},
+		}
+		require.True(t, oauthAccount.IsOpenAIOAuthPassthroughEnabled())
+
+		apiKeyAccount := &Account{
+			Platform: PlatformOpenAI,
+			Type:     AccountTypeAPIKey,
+			Extra: map[string]any{
+				"openai_passthrough": true,
+			},
+		}
+		require.False(t, apiKeyAccount.IsOpenAIOAuthPassthroughEnabled())
+	})
+}