refactor(vscode-ide-companion): simplify openDiff tool implementation

- Remove redundant file reading logic from ide-server
- Leverage diffManager's new capability to read old content internally
- Simplify openDiff tool call site to pass only newContent
- Update comments to reflect the simplified implementation

packages/vscode-ide-companion/src/ide-server.ts

@@ -437,18 +437,8 @@ const createMcpServer = (diffManager: DiffManager) => {
       inputSchema: OpenDiffRequestSchema.shape,
     },
     async ({ filePath, newContent }: z.infer<typeof OpenDiffRequestSchema>) => {
-      // Read old content if file exists, otherwise use empty string
-      let oldContent = '';
-      try {
-        const fileUri = vscode.Uri.file(filePath);
-        const document = await vscode.workspace.openTextDocument(fileUri);
-        oldContent = document.getText();
-      } catch (_error) {
-        // File doesn't exist, use empty string (creating new file)
-        oldContent = '';
-      }
-
-      await diffManager.showDiff(filePath, oldContent, newContent);
+      // Minimal call site: only pass newContent; DiffManager reads old content itself
+      await diffManager.showDiff(filePath, newContent);
       return { content: [] };
     },
   );