feat: Export diagram from model to mermaid code

[lornakelly]

Description

Add support for exporting Mermaid diagrams from the workflow's JSON/YAML model using the CNCF TypeScript SDK's convertToMermaidCode class. This approach deserialises the workflow model directly and converts it to Mermaid code.

Building a custom React Flow → Mermaid mapper was evaluated but ruled out due to high complexity and maintenance burden. The CNCF SDK approach was selected as the best balance of maintainability and implementation effort.

Motivation

Users want to retain Mermaid output for documentation purposes

Proposed Implementation

Design TBD

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.