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/service/account_test_service.go

@@ -564,7 +564,7 @@ func (s *AccountTestService) testOpenAIAccountConnection(c *gin.Context, account
 					"账号本身可正常使用，但当前测试接口仅支持 Responses API 路径。请直接通过实际 API 调用验证。",
 			)
 		}
-		apiURL = strings.TrimSuffix(normalizedBaseURL, "/") + "/responses"
+		apiURL = buildOpenAIResponsesURL(normalizedBaseURL)
 	} else {
 		return s.sendErrorAndEnd(c, fmt.Sprintf("Unsupported account type: %s", account.Type))
 	}