fix(openai-gateway): address PR review — probe URL /v1 prefix, Create trigger, tests

Fix four issues flagged by copilot-pull-request-reviewer on PR #2143:

1. Probe URL missing /v1 prefix (openai_apikey_responses_probe.go)
   Replaced bare TrimSuffix + "/responses" with buildOpenAIResponsesURL(),
   which handles bare domain → /v1/responses correctly. Affected:
   - ProbeOpenAIAPIKeyResponsesSupport (probe URL)
   - TestAccount endpoint (apiURL for APIKey accounts)

2. Create endpoint not triggering probe (account_handler.go)
   Capture created account from idempotent closure and call
   scheduleOpenAIResponsesProbe after success, same pattern as
   BatchCreate and Update.

3. Tests (openai_gateway_chat_completions_raw_test.go)
   Added TestBuildOpenAIChatCompletionsURL (7 cases covering
   bare domain, /v1 suffix, trailing slash, third-party domains,
   whitespace) and TestBuildOpenAIResponsesURL_ProbeURL (6 cases
   locking the probe URL construction for bare-domain inputs).

All unit tests pass; go build ./cmd/server/ clean.

backend/internal/handler/admin/account_handler.go

@@ -528,6 +528,10 @@ func (h *AccountHandler) Create(c *gin.Context) {
 	// 确定是否跳过混合渠道检查
 	skipCheck := req.ConfirmMixedChannelRisk != nil && *req.ConfirmMixedChannelRisk
 
+	// 捕获闭包内创建的账号引用，用于创建成功后触发异步探测。
+	// 幂等重放时闭包不会执行 → createdAccount 为 nil → 不重复调度。
+	var createdAccount *service.Account
+
 	result, err := executeAdminIdempotent(c, "admin.accounts.create", req, service.DefaultWriteIdempotencyTTL(), func(ctx context.Context) (any, error) {
 		account, execErr := h.adminService.CreateAccount(ctx, &service.CreateAccountInput{
 			Name:                  req.Name,
@@ -549,6 +553,7 @@ func (h *AccountHandler) Create(c *gin.Context) {
 		if execErr != nil {
 			return nil, execErr
 		}
+		createdAccount = account
 		// Antigravity OAuth: 新账号直接设置隐私
 		h.adminService.ForceAntigravityPrivacy(ctx, account)
 		// OpenAI OAuth: 新账号直接设置隐私
@@ -577,6 +582,9 @@ func (h *AccountHandler) Create(c *gin.Context) {
 	if result != nil && result.Replayed {
 		c.Header("X-Idempotency-Replayed", "true")
 	}
+	// OpenAI APIKey 账号创建后异步探测上游 /v1/responses 能力。
+	// 探测失败不影响账号创建响应。
+	h.scheduleOpenAIResponsesProbe(createdAccount)
 	response.Success(c, result.Data)
 }