feat: Verify and Enhance Workflow-Level Validation

[lornakelly]

Description

Verify that the existing workflow-level validation (via validateWorkflow in workflowSdk.ts) correctly validates the full workflow document and provides comprehensive error reporting. Enhance the validation integration to ensure all workflow-level rules are properly surfaced to users.

Motivation

The editor already implements workflow-level validation through the SDK's validate("Workflow", model) function in workflowSdk.ts. We need to verify this validation and identify if improvements need to be made

Proposed Implementation

Refer to spike #284

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.