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))
 	}

backend/internal/service/openai_apikey_responses_probe.go

@@ -85,7 +85,7 @@ func (s *AccountTestService) ProbeOpenAIAPIKeyResponsesSupport(ctx context.Conte
 		return
 	}
 
-	probeURL := strings.TrimSuffix(normalizedBaseURL, "/") + "/responses"
+	probeURL := buildOpenAIResponsesURL(normalizedBaseURL)
 
 	probeCtx, cancel := context.WithTimeout(ctx, openaiResponsesProbeTimeout)
 	defer cancel()

backend/internal/service/openai_gateway_chat_completions_raw_test.go

@@ -0,0 +1,67 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestBuildOpenAIChatCompletionsURL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		base string
+		want string
+	}{
+		// 已是 /chat/completions：原样返回
+		{"already chat/completions", "https://api.openai.com/v1/chat/completions", "https://api.openai.com/v1/chat/completions"},
+		// 以 /v1 结尾：追加 /chat/completions
+		{"bare /v1", "https://api.openai.com/v1", "https://api.openai.com/v1/chat/completions"},
+		// 其他情况：追加 /v1/chat/completions
+		{"bare domain", "https://api.openai.com", "https://api.openai.com/v1/chat/completions"},
+		{"domain with trailing slash", "https://api.openai.com/", "https://api.openai.com/v1/chat/completions"},
+		// 第三方上游常见形式
+		{"third-party bare domain", "https://api.deepseek.com", "https://api.deepseek.com/v1/chat/completions"},
+		{"third-party with path prefix", "https://api.gptgod.online/api", "https://api.gptgod.online/api/v1/chat/completions"},
+		// 带空白字符
+		{"whitespace trimmed", "  https://api.openai.com/v1  ", "https://api.openai.com/v1/chat/completions"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got := buildOpenAIChatCompletionsURL(tt.base)
+			require.Equal(t, tt.want, got)
+		})
+	}
+}
+
+// TestBuildOpenAIResponsesURL_ProbeURL 锁定 probe/测试端点使用的 URL 构建逻辑，
+// 确保 buildOpenAIResponsesURL 对标准 OpenAI base_url 格式均拼出 `/v1/responses`。
+func TestBuildOpenAIResponsesURL_ProbeURL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		base string
+		want string
+	}{
+		{"bare domain", "https://api.openai.com", "https://api.openai.com/v1/responses"},
+		{"domain trailing slash", "https://api.openai.com/", "https://api.openai.com/v1/responses"},
+		{"bare /v1", "https://api.openai.com/v1", "https://api.openai.com/v1/responses"},
+		{"already /responses", "https://api.openai.com/v1/responses", "https://api.openai.com/v1/responses"},
+		{"third-party bare domain", "https://api.deepseek.com", "https://api.deepseek.com/v1/responses"},
+		{"only domain, no scheme", "api.gptgod.online", "api.gptgod.online/v1/responses"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got := buildOpenAIResponsesURL(tt.base)
+			require.Equal(t, tt.want, got)
+		})
+	}
+}