feat: Update React Flow builder to support parent/child nesting

[lornakelly]

Description

Update the React Flow builder to support parent-child nesting when constructing the Node[] array. Child nodes should be assigned a parentId, and their positions calculated relative to their parent node rather than using absolute coordinates. This ensures correct rendering and alignment of nested structures within composite task containers.

Motivation

Enable accurate visual representation of nested workflows by aligning the underlying node construction with React Flow’s parent–child model, ensuring consistency between data structure and diagram layout

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.