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/handler/openai_gateway_handler.go

@@ -13,13 +13,11 @@ import (
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/ip"
-	"github.com/Wei-Shaw/sub2api/internal/pkg/openai"
 	middleware2 "github.com/Wei-Shaw/sub2api/internal/server/middleware"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 
 	"github.com/gin-gonic/gin"
 	"github.com/tidwall/gjson"
-	"github.com/tidwall/sjson"
 )
 
 // OpenAIGatewayHandler handles OpenAI API gateway requests
@@ -118,22 +116,6 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 	}
 	reqStream := streamResult.Bool()
 
-	userAgent := c.GetHeader("User-Agent")
-	isCodexCLI := openai.IsCodexCLIRequest(userAgent) || (h.cfg != nil && h.cfg.Gateway.ForceCodexCLI)
-	if !isCodexCLI {
-		existingInstructions := gjson.GetBytes(body, "instructions").String()
-		if strings.TrimSpace(existingInstructions) == "" {
-			if instructions := strings.TrimSpace(service.GetOpenCodeInstructions()); instructions != "" {
-				newBody, err := sjson.SetBytes(body, "instructions", instructions)
-				if err != nil {
-					h.errorResponse(c, http.StatusInternalServerError, "api_error", "Failed to process request")
-					return
-				}
-				body = newBody
-			}
-		}
-	}
-
 	setOpsRequestContext(c, reqModel, reqStream, body)
 
 	// 提前校验 function_call_output 是否具备可关联上下文，避免上游 400。