refactor(antigravity): 提取并同步 Schema 清理逻辑至 schema_cleaner.go

主要变更：
1. 重构代码结构：
   - 将 CleanJSONSchema 及其相关辅助函数从 request_transformer.go 提取到独立的 schema_cleaner.go 文件中，实现逻辑解耦。

2. 逻辑优化与修正：
   - 参考 Antigravity-Manager (json_schema.rs) 的实现逻辑，修正了 Schema 清洗策略。

backend/internal/pkg/antigravity/response_transformer.go

@@ -3,6 +3,7 @@ package antigravity
 import (
 	"encoding/json"
 	"fmt"
+	"log"
 	"strings"
 )
 
@@ -242,6 +243,14 @@ func (p *NonStreamingProcessor) buildResponse(geminiResp *GeminiResponse, respon
 	var finishReason string
 	if len(geminiResp.Candidates) > 0 {
 		finishReason = geminiResp.Candidates[0].FinishReason
+		if finishReason == "MALFORMED_FUNCTION_CALL" {
+			log.Printf("[Antigravity] MALFORMED_FUNCTION_CALL detected in response for model %s", originalModel)
+			if geminiResp.Candidates[0].Content != nil {
+				if b, err := json.Marshal(geminiResp.Candidates[0].Content); err == nil {
+					log.Printf("[Antigravity] Malformed content: %s", string(b))
+				}
+			}
+		}
 	}
 
 	stopReason := "end_turn"