Add tests for tidier.js

[Geethegreat]

Fixes #3816

Changes:
Adds Jest test coverage for tidier.js to verify:

• Running “Tidy Code” updates the editor document using prettier.formatWithCursor
• The cursor position is preserved and updated correctly
• Formatting works for JavaScript, CSS, and HTML modes
• Unsupported modes do not trigger formatting
  Prettier is mocked to ensure the tests validate our integration behavior rather than Prettier internals.

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, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines