feat: Improve file path handling in contextFileSelected

- Add fallback for relative path calculation
- Handle cases where workspaceDir is undefined
- Ensure fileSelected is always used as a fallback

Author: yangbobo2021

src/context/contextFileSelected.ts

@@ -19,7 +19,9 @@ export async function handleFileSelected(fileSelected: string) {
 
 	// get relative path of workspace
 	const workspaceDir = UiUtilWrapper.workspaceFoldersFirstPath();
-	const relativePath = path.relative(workspaceDir!, fileSelected);
+	const relativePath = workspaceDir 
+        ? path.relative(workspaceDir, fileSelected)
+        : fileSelected;
 
     // convert fileContent to markdown code block with languageId and file path
     const data = {