fix: Antigravity 测试连接使用最小 token 消耗

- buildGeminiTestRequest: 输入 "." + maxOutputTokens: 1
- buildClaudeTestRequest: 输入 "." + MaxTokens: 1
- buildGenerationConfig: 支持透传 MaxTokens 参数

backend/internal/pkg/antigravity/request_transformer.go

@@ -429,6 +429,11 @@ func buildGenerationConfig(req *ClaudeRequest) *GeminiGenerationConfig {
 		StopSequences:   DefaultStopSequences,
 	}
 
+	// 如果请求中指定了 MaxTokens，使用请求值
+	if req.MaxTokens > 0 {
+		config.MaxOutputTokens = req.MaxTokens
+	}
+
 	// Thinking 配置
 	if req.Thinking != nil && req.Thinking.Type == "enabled" {
 		config.ThinkingConfig = &GeminiThinkingConfig{

backend/internal/service/antigravity_gateway_service.go

@@ -276,13 +276,14 @@ func (s *AntigravityGatewayService) TestConnection(ctx context.Context, account
 }
 
 // buildGeminiTestRequest 构建 Gemini 格式测试请求
+// 使用最小 token 消耗：输入 "." + maxOutputTokens: 1
 func (s *AntigravityGatewayService) buildGeminiTestRequest(projectID, model string) ([]byte, error) {
 	payload := map[string]any{
 		"contents": []map[string]any{
 			{
 				"role": "user",
 				"parts": []map[string]any{
-					{"text": "hi"},
+					{"text": "."},
 				},
 			},
 		},
@@ -292,22 +293,26 @@ func (s *AntigravityGatewayService) buildGeminiTestRequest(projectID, model stri
 				{"text": antigravity.GetDefaultIdentityPatch()},
 			},
 		},
+		"generationConfig": map[string]any{
+			"maxOutputTokens": 1,
+		},
 	}
 	payloadBytes, _ := json.Marshal(payload)
 	return s.wrapV1InternalRequest(projectID, model, payloadBytes)
 }
 
 // buildClaudeTestRequest 构建 Claude 格式测试请求并转换为 Gemini 格式
+// 使用最小 token 消耗：输入 "." + MaxTokens: 1
 func (s *AntigravityGatewayService) buildClaudeTestRequest(projectID, mappedModel string) ([]byte, error) {
 	claudeReq := &antigravity.ClaudeRequest{
 		Model: mappedModel,
 		Messages: []antigravity.ClaudeMessage{
 			{
 				Role:    "user",
-				Content: json.RawMessage(`"hi"`),
+				Content: json.RawMessage(`"."`),
 			},
 		},
-		MaxTokens: 1024,
+		MaxTokens: 1,
 		Stream:    false,
 	}
 	return antigravity.TransformClaudeToGemini(claudeReq, projectID, mappedModel)