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/geminicli_codeassist_client.go

@@ -26,7 +26,11 @@ func (c *geminiCliCodeAssistClient) LoadCodeAssist(ctx context.Context, accessTo
 	}
 
 	var out geminicli.LoadCodeAssistResponse
-	resp, err := createGeminiCliReqClient(proxyURL).R().
+	client, err := createGeminiCliReqClient(proxyURL)
+	if err != nil {
+		return nil, fmt.Errorf("create HTTP client: %w", err)
+	}
+	resp, err := client.R().
 		SetContext(ctx).
 		SetHeader("Authorization", "Bearer "+accessToken).
 		SetHeader("Content-Type", "application/json").
@@ -66,7 +70,11 @@ func (c *geminiCliCodeAssistClient) OnboardUser(ctx context.Context, accessToken
 	fmt.Printf("[CodeAssist] OnboardUser request body: %+v\n", reqBody)
 
 	var out geminicli.OnboardUserResponse
-	resp, err := createGeminiCliReqClient(proxyURL).R().
+	client, err := createGeminiCliReqClient(proxyURL)
+	if err != nil {
+		return nil, fmt.Errorf("create HTTP client: %w", err)
+	}
+	resp, err := client.R().
 		SetContext(ctx).
 		SetHeader("Authorization", "Bearer "+accessToken).
 		SetHeader("Content-Type", "application/json").
@@ -98,7 +106,7 @@ func (c *geminiCliCodeAssistClient) OnboardUser(ctx context.Context, accessToken
 	return &out, nil
 }
 
-func createGeminiCliReqClient(proxyURL string) *req.Client {
+func createGeminiCliReqClient(proxyURL string) (*req.Client, error) {
 	return getSharedReqClient(reqClientOptions{
 		ProxyURL: proxyURL,
 		Timeout:  30 * time.Second,