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/repository/gemini_oauth_client.go

@@ -26,7 +26,10 @@ func NewGeminiOAuthClient(cfg *config.Config) service.GeminiOAuthClient {
 }
 
 func (c *geminiOAuthClient) ExchangeCode(ctx context.Context, oauthType, code, codeVerifier, redirectURI, proxyURL string) (*geminicli.TokenResponse, error) {
-	client := createGeminiReqClient(proxyURL)
+	client, err := createGeminiReqClient(proxyURL)
+	if err != nil {
+		return nil, fmt.Errorf("create HTTP client: %w", err)
+	}
 
 	// Use different OAuth clients based on oauthType:
 	// - code_assist: always use built-in Gemini CLI OAuth client (public)
@@ -72,7 +75,10 @@ func (c *geminiOAuthClient) ExchangeCode(ctx context.Context, oauthType, code, c
 }
 
 func (c *geminiOAuthClient) RefreshToken(ctx context.Context, oauthType, refreshToken, proxyURL string) (*geminicli.TokenResponse, error) {
-	client := createGeminiReqClient(proxyURL)
+	client, err := createGeminiReqClient(proxyURL)
+	if err != nil {
+		return nil, fmt.Errorf("create HTTP client: %w", err)
+	}
 
 	oauthCfgInput := geminicli.OAuthConfig{
 		ClientID:     c.cfg.Gemini.OAuth.ClientID,
@@ -111,7 +117,7 @@ func (c *geminiOAuthClient) RefreshToken(ctx context.Context, oauthType, refresh
 	return &tokenResp, nil
 }
 
-func createGeminiReqClient(proxyURL string) *req.Client {
+func createGeminiReqClient(proxyURL string) (*req.Client, error) {
 	return getSharedReqClient(reqClientOptions{
 		ProxyURL: proxyURL,
 		Timeout:  60 * time.Second,