Add REST API endpoints for creating and managing entity-context playground sessions

Summary

Implement RESTful API endpoints that allow programmatic creation of playground sessions with entity context. This enables external systems, custom modules, and JavaScript applications to create entity-aware workflow sessions without requiring direct PHP access.

Business Value

• Enables decoupled frontends to launch FlowDrop workflows for specific entities
• Supports integration with external content management systems and workflows
• Allows building custom UI components that trigger entity-specific workflows
• Provides programmatic access for batch processing and automation scripts

Features Implemented

• Create Session Endpoint: POST endpoint to create sessions with entity context
• Get Entity Context Endpoint: GET endpoint to retrieve entity context from existing sessions
• JSON Request/Response: Standard JSON format for all API interactions
• Comprehensive Error Handling: Proper HTTP status codes and error messages

Technical Implementation

• NodeSessionApiController handling all REST endpoints
• Route: POST /api/flowdrop-node-session/workflows/{workflow_id}/sessions
• Route: GET /api/flowdrop-node-session/sessions/{session_id}/entity-context
• Custom access callbacks for fine-grained permission control
• Request body parsing with validation for required parameters

API Usage Example

POST /api/flowdrop-node-session/workflows/my_workflow/sessions
Content-Type: application/json

{
  "entity_type": "node",
  "entity_id": "123",
  "revision_id": "456",
  "bundle": "article",
  "name": "Review Article #123",
  "metadata": {}
}