feat: Add CatchContainer to graph node types #141

Description

The SDK currently exposes a single Catch entry in GraphNodeType, however, a catch block in the DSL has two distinct visual shapes depending on its content:

1. Leaf - when catch.do is absent, the catch is a single handler with no nested flow.
2. Container - when catch.do is present, the catch wraps a sequence of child tasks that run when the error is caught, and should render as a container with its own internal subgraph.

The current single Catch type can't represent both. We need to extend GraphNodeType to add a CatchContainer variant alongside the existing leaf Catch, and route to the correct one at graph-build time based on the presence of catch.do

Motivation

• Correct visual representation - a catch with nested tasks is structurally a container (it has an internal flow), and rendering it as a leaf hides that flow from the user. They have to read the YAML to know there's anything inside.

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.