feat(proxy): 集中代理 URL 验证并实现全局 fail-fast

提取 proxyurl.Parse() 公共包，将分散在 6 处的代理 URL 验证逻辑
统一收敛，确保无效代理配置在创建时立即失败，永不静默回退直连。

主要变更：
- 新增 proxyurl 包：统一 TrimSpace → url.Parse → Host 校验 → Scheme 白名单
- socks5:// 自动升级为 socks5h://，防止 DNS 泄漏（大小写不敏感）
- antigravity: http.ProxyURL → proxyutil.ConfigureTransportProxy 支持 SOCKS5
- openai_oauth: 删除 newOpenAIOAuthHTTPClient，收编至 httpclient.GetClient
- 移除未使用的 ProxyStrict 字段（fail-fast 已是全局默认行为）
- 补充 15 个 proxyurl 测试 + pricing/usage fail-fast 测试

backend/internal/service/openai_oauth_service.go

@@ -7,7 +7,6 @@ import (
 	"io"
 	"log/slog"
 	"net/http"
-	"net/url"
 	"regexp"
 	"sort"
 	"strconv"
@@ -15,6 +14,7 @@ import (
 	"time"
 
 	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/httpclient"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/openai"
 )
 
@@ -273,7 +273,13 @@ func (s *OpenAIOAuthService) ExchangeSoraSessionToken(ctx context.Context, sessi
 	req.Header.Set("Referer", "https://sora.chatgpt.com/")
 	req.Header.Set("User-Agent", "Sora/1.2026.007 (Android 15; 24122RKC7C; build 2600700)")
 
-	client := newOpenAIOAuthHTTPClient(proxyURL)
+	client, err := httpclient.GetClient(httpclient.Options{
+		ProxyURL: proxyURL,
+		Timeout:  120 * time.Second,
+	})
+	if err != nil {
+		return nil, infraerrors.Newf(http.StatusBadGateway, "SORA_SESSION_CLIENT_FAILED", "create http client failed: %v", err)
+	}
 	resp, err := client.Do(req)
 	if err != nil {
 		return nil, infraerrors.Newf(http.StatusBadGateway, "SORA_SESSION_REQUEST_FAILED", "request failed: %v", err)
@@ -530,19 +536,6 @@ func (s *OpenAIOAuthService) resolveProxyURL(ctx context.Context, proxyID *int64
 	return proxy.URL(), nil
 }
 
-func newOpenAIOAuthHTTPClient(proxyURL string) *http.Client {
-	transport := &http.Transport{}
-	if strings.TrimSpace(proxyURL) != "" {
-		if parsed, err := url.Parse(proxyURL); err == nil && parsed.Host != "" {
-			transport.Proxy = http.ProxyURL(parsed)
-		}
-	}
-	return &http.Client{
-		Timeout:   120 * time.Second,
-		Transport: transport,
-	}
-}
-
 func normalizeOpenAIOAuthPlatform(platform string) string {
 	switch strings.ToLower(strings.TrimSpace(platform)) {
 	case PlatformSora: