feat: Preserve UTF-8 BOM when editing files (Fix #1672)

- Add FileEncoding constants (UTF8, UTF8_BOM)
- Add detectFileBOM() to detect existing file encoding
- Modify writeTextFile() to support BOM option
- Add defaultFileEncoding configuration option
- Preserve BOM when editing existing files
- Use configured encoding for new files
- Add comprehensive tests (unit, integration, e2e)
- Update documentation

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/src/acp-integration/service/filesystem.ts

@@ -54,17 +54,30 @@ export class AcpFileSystemService implements FileSystemService {
     return response.content;
   }
 
-  async writeTextFile(filePath: string, content: string): Promise<void> {
+  async writeTextFile(
+    filePath: string,
+    content: string,
+    options?: { bom?: boolean },
+  ): Promise<void> {
     if (!this.capabilities.writeTextFile) {
-      return this.fallback.writeTextFile(filePath, content);
+      return this.fallback.writeTextFile(filePath, content, options);
     }
 
+    // Prepend BOM character if requested
+    const finalContent = options?.bom ? '\uFEFF' + content : content;
+
     await this.client.writeTextFile({
       path: filePath,
-      content,
+      content: finalContent,
       sessionId: this.sessionId,
     });
   }
+
+  async detectFileBOM(filePath: string): Promise<boolean> {
+    // Always use fallback for BOM detection
+    return this.fallback.detectFileBOM(filePath);
+  }
+
   findFiles(fileName: string, searchPaths: readonly string[]): string[] {
     return this.fallback.findFiles(fileName, searchPaths);
   }