fix(openai-oauth): 改进错误处理和代理支持

- 使用 ApplicationError 返回详细错误信息到前端
- 添加 User-Agent: codex-cli/0.91.0
- 移除 ForceHTTP2 以兼容 HTTP 代理
- 修复代理获取失败时静默忽略的问题
- 500 错误时记录完整错误日志

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/pkg/response/response.go

@@ -2,6 +2,7 @@
 package response
 
 import (
+	"log"
 	"math"
 	"net/http"
 
@@ -74,6 +75,12 @@ func ErrorFrom(c *gin.Context, err error) bool {
 	}
 
 	statusCode, status := infraerrors.ToHTTP(err)
+
+	// Log internal errors with full details for debugging
+	if statusCode >= 500 {
+		log.Printf("[ERROR] %s %s\n  Error: %s", c.Request.Method, c.Request.URL.Path, err.Error())
+	}
+
 	ErrorWithDetails(c, statusCode, status.Message, status.Reason, status.Metadata)
 	return true
 }