将所有agent工具的explanation字段改为可选

修复Pydantic验证错误：QueryTransferHistoryInput的explanation字段为必需，但用户未提供。
修改了74个工具文件，将explanation字段从必需改为可选，默认值为None。

app/agent/tools/impl/scrape_metadata.py

@@ -15,10 +15,8 @@ from app.schemas import FileItem
 class ScrapeMetadataInput(BaseModel):
     """刮削媒体元数据工具的输入参数模型"""
 
-    explanation: str = Field(
-        ...,
-        description="Clear explanation of why this tool is being used in the current context",
-    )
+    explanation: Optional[str] = Field(None,
+        description="Clear explanation of why this tool is being used in the current context",)
     path: str = Field(
         ...,
         description="Path to the file or directory to scrape metadata for (e.g., '/path/to/file.mkv' or '/path/to/directory')",