feat: Render SDK validation errors

[lornakelly]

Description

When the SDK reports validation errors but the workflow is still structurally renderable i.e the model in store is not null, render the diagram and display the validation errors to the user. This will require parsing the errors returned by the SDK, storing them in state, and showing them in the UI, initially in the top-right area of the screen. Styling will be applied at a later stage, simple for now but will need an overlay for canvas (in React flow docs)

Motivation

Allow users to continue viewing and understanding the workflow even when validation issues exist, while still making those issues visible.

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.