修复: FR-S2 read_file TOCTOU + write_file 无大小限制

- read_file: 单次 File::open 取 metadata+read 消除 TOCTOU(原 metadata+read 两步 syscall 间 symlink 替换破 1MB 限)
- write_file: content.len() 超 1MB bail(原无限制,LLM 经审批可写超大文件撑爆磁盘/对话历史)
来源 fullstack-review §2 P0 安全(已核实);cargo 0 err

src-tauri/src/commands/ai/tool_registry.rs

@@ -361,12 +361,18 @@ pub fn build_ai_tool_registry(db: &Arc<Database>) -> AiToolRegistry {
                 args["path"].as_str().ok_or_else(|| anyhow::anyhow!("缺少 path 参数"))?,
             )?;
             let path = resolved.to_str().ok_or_else(|| anyhow::anyhow!("路径含非法字符"))?;
-            let metadata = tokio::fs::metadata(path).await
+            // 单次 File::open 取 metadata+read 消除 TOCTOU(FR-S2:原 metadata+read 两步 syscall 间 symlink 替换破 1MB 限)
+            use tokio::fs::File;
+            use tokio::io::AsyncReadExt;
+            let mut file = File::open(path).await
                 .map_err(|e| anyhow::anyhow!("无法访问文件 {}: {}", path, e))?;
+            let metadata = file.metadata().await
+                .map_err(|e| anyhow::anyhow!("读取元数据失败 {}: {}", path, e))?;
             if metadata.len() > 1_048_576 {
                 anyhow::bail!("文件超过 1MB 限制 ({} 字节)", metadata.len());
             }
-            let content = tokio::fs::read_to_string(path).await
+            let mut content = String::new();
+            file.read_to_string(&mut content).await
                 .map_err(|e| anyhow::anyhow!("读取文件失败: {}", e))?;
             let result = if let Some(offset) = args["offset"].as_u64() {
                 let lines: Vec<&str> = content.lines().collect();
@@ -406,6 +412,10 @@ pub fn build_ai_tool_registry(db: &Arc<Database>) -> AiToolRegistry {
             )?;
             let path = resolved.to_str().ok_or_else(|| anyhow::anyhow!("路径含非法字符"))?;
             let content = args["content"].as_str().ok_or_else(|| anyhow::anyhow!("缺少 content 参数"))?;
+            // 写入大小上限(FR-S2:原无限制,LLM 经审批可写超大文件撑爆磁盘/对话历史)
+            if content.len() > 1_048_576 {
+                anyhow::bail!("写入内容超过 1MB 限制 ({} 字节)", content.len());
+            }
             if let Some(parent) = std::path::Path::new(path).parent() {
                 tokio::fs::create_dir_all(parent).await
                     .map_err(|e| anyhow::anyhow!("创建目录失败: {}", e))?;