feat: Increase unit test coverage across core modules

[lornakelly]

Description

Review current unit test coverage across core modules and expand it where needed. Focus on strengthening coverage for key logic and areas with limited or missing tests.

Motivation

Improve confidence in core behaviour and reduce the risk of regressions as the editor evolves.

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.