[FEATURE] Establish and Implement Cypress Testing Best Practices

### Certification

- [x] I certify I am an Epic Owner for Kubeflow Notebooks 2.0 and expected to create planning-related issues.

### Motivation

This feature will help users by:

- Improving test maintainability by using consistent, stable selectors that are less likely to break when UI styling changes
- Reducing potential for test flakiness by following Cypress-recommended practices for element selection
- Creating clear guidelines that all team members can follow when writing new tests

### High Level Design / Mock-ups

See https://docs.cypress.io/app/core-concepts/best-practices

Start by reading the linked Cypress best-practices documentation. Then locate the repository's existing Cypress tests and compare their selectors and patterns with the guidance. Done requires agreed, documented conventions and a clear scope for applying them, but this issue does not name specific files or tests.

Written by the indexing model from the issue text.