feat: Surface validation errors in the diagram UI

[lornakelly]

Description

Dependencies:
#96
#144

Focus on 2 areas:

• Node styling - apply an error state to nodes that have associated validation errors (red border + ! badge as discussed).
• Sidebar errors group - when an erroring node is selected, display the structured errors from state in the sidepanel above the node properties with agreed styling

Motivation

Allow users to continue viewing and understanding their workflow even when validation issues exist, while making those issues clearly visible both on the canvas and in the sidebar.

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.