feat: Review and increase E2E test coverage

[lornakelly]

Description

Review existing Playwright end-to-end tests and expand coverage where needed. Focus on adding tests for key user flows and diagram rendering scenarios to ensure reliable behaviour across the application.

Motivation

Strengthen confidence in end-to-end behaviour and reduce the risk of regressions in critical user interactions.

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.