fix(filesystem): ensure bare Windows drive letters normalize to root (#3434)

fix(filesystem): ensure bare Windows drive letters normalize to root

Appends path.sep to bare drive letters (e.g. "C:") before calling path.normalize(), preventing them from normalizing to "C:." (current directory on drive) instead of "C:\" (drive root). Includes test coverage with platform mocking.

Fixes #3418

src/filesystem/path-utils.ts

@@ -77,6 +77,12 @@ export function normalizePath(p: string): string {
     p = p.replace(/\\\\/g, '\\');
   }
 
+  // On Windows, if we have a bare drive letter (e.g. "C:"), append a separator
+  // so path.normalize doesn't return "C:." which can break path validation.
+  if (process.platform === 'win32' && /^[a-zA-Z]:$/.test(p)) {
+    p = p + path.sep;
+  }
+
   // Use Node's path normalization, which handles . and .. segments
   let normalized = path.normalize(p);
 
@@ -116,3 +122,4 @@ export function expandHome(filepath: string): string {
   }
   return filepath;
 }
+