Add tests for file deletion in files reducer

[aashu2006]

Changes:
Added reducer tests for the DELETE_FILE action in client/modules/IDE/reducers/files.js.

The tests verify that file and folder deletion works correctly, including nested and empty folders, and that other files remain unchanged. No production code was modified.

Accessibility: Not applicable, this PR adds tests only and does not modify UI, markup, or styles.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number
• meets the standards outlined in the accessibility guidelines

[aashu2006]

Taking up @clairep94 suggestion to add more test coverage to the web editor. Beginning with the file deletion logic in the files reducer. 🙂