feat: Create and style entry/exit indicators for container nodes #140

Dependency: #80

Description

Container nodes wrap a child workflow that has its own internal flow. To preserve encapsulation, no child node inside a container should connect directly to anything outside the parent container - instead, the container itself exposes entry and exit points that act as the only connection surface between the container's internals and the rest of the graph.

Motivation

• Visual clarity - users can tell at a glance where flow enters and leaves a container without tracing edges through nested children.
• Encapsulation - prevents "leaky" edges that bypass the container's semantics (e.g. a child of a try connecting directly to a node after the try, which would misrepresent the DSL).
• Correct DSL mapping - the Serverless Workflow DSL treats containers as single units of control flow; entry/exit indicators make this explicit in the diagram and keep the visual model aligned with the underlying YAML.

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.