feat(account): 账号测试支持选择模型

- 新增 GET /api/v1/admin/accounts/:id/models 接口获取账号可用模型
- 账号测试弹窗新增模型选择下拉框
- 测试时支持传入 model_id 参数，不传则默认使用 Sonnet
- API Key 账号支持根据 model_mapping 映射测试模型
- 将模型常量提取到 claude 包统一管理

backend/internal/service/account_test_service.go

@@ -24,7 +24,6 @@ import (
 
 const (
 	testClaudeAPIURL = "https://api.anthropic.com/v1/messages"
-	testModel        = "claude-sonnet-4-5-20250929"
 )
 
 // TestEvent represents a SSE event for account testing
@@ -64,9 +63,9 @@ func generateSessionString() string {
 }
 
 // createTestPayload creates a Claude Code style test request payload
-func createTestPayload() map[string]interface{} {
+func createTestPayload(modelID string) map[string]interface{} {
 	return map[string]interface{}{
-		"model": testModel,
+		"model": modelID,
 		"messages": []map[string]interface{}{
 			{
 				"role": "user",
@@ -101,7 +100,8 @@ func createTestPayload() map[string]interface{} {
 
 // TestAccountConnection tests an account's connection by sending a test request
 // All account types use full Claude Code client characteristics, only auth header differs
-func (s *AccountTestService) TestAccountConnection(c *gin.Context, accountID int64) error {
+// modelID is optional - if empty, defaults to claude.DefaultTestModel
+func (s *AccountTestService) TestAccountConnection(c *gin.Context, accountID int64, modelID string) error {
 	ctx := c.Request.Context()
 
 	// Get account
@@ -110,6 +110,22 @@ func (s *AccountTestService) TestAccountConnection(c *gin.Context, accountID int
 		return s.sendErrorAndEnd(c, "Account not found")
 	}
 
+	// Determine the model to use
+	testModelID := modelID
+	if testModelID == "" {
+		testModelID = claude.DefaultTestModel
+	}
+
+	// For API Key accounts with model mapping, map the model
+	if account.Type == "apikey" {
+		mapping := account.GetModelMapping()
+		if mapping != nil && len(mapping) > 0 {
+			if mappedModel, exists := mapping[testModelID]; exists {
+				testModelID = mappedModel
+			}
+		}
+	}
+
 	// Determine authentication method and API URL
 	var authToken string
 	var useBearer bool
@@ -165,11 +181,11 @@ func (s *AccountTestService) TestAccountConnection(c *gin.Context, accountID int
 	c.Writer.Flush()
 
 	// Create Claude Code style payload (same for all account types)
-	payload := createTestPayload()
+	payload := createTestPayload(testModelID)
 	payloadBytes, _ := json.Marshal(payload)
 
 	// Send test_start event
-	s.sendEvent(c, TestEvent{Type: "test_start", Model: testModel})
+	s.sendEvent(c, TestEvent{Type: "test_start", Model: testModelID})
 
 	req, err := http.NewRequestWithContext(ctx, "POST", apiURL, bytes.NewReader(payloadBytes))
 	if err != nil {