feat: Investigate and improve SDK validation error messaging

[lornakelly]

Description

Make improvements to the @openworkflowspec/sdk TypeScript package itself to provide better validation error messages and structured error output. Leverage AJV's built-in capabilities for custom error messages and improve the SDK's error formatting to reduce the need for complex parsing in consuming applications.

Motivation

The current error messages are verbose and not user-friendly, making it difficult for users to understand what went wrong and how to fix it. The SDK uses AJV for validation, which has powerful features for defining meaningful, user-friendly error messages that are currently underutilised.

Examples of current messages

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Open 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.