feat: Add error boundary for diagram rendering

[lornakelly]

Description

Introduce an error boundary around the diagram editor component to catch unexpected errors in the UI. This is distinct from content validation failures - it handles unforeseen issues during rendering (e.g. React or React Flow crashes) rather than invalid workflow input. When such an error occurs, display a fallback UI instead of allowing the editor to crash or render a blank screen.

Motivation

Improve overall stability and resilience of the editor by safely handling unexpected rendering failures and preventing full UI crashes.

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.