Implement GET /areas/{area_id}/components endpoint to list components within a specific area

[mfaferek93]

Feature request / General issue

Implement GET /areas/{area_id}/components endpoint to list components within a specific area with proper error handling for nonexistent areas.

Proposal

Add a REST API endpoint at GET /areas/{area_id}/components that returns all components belonging to a specific area. The endpoint should filter components by area from the
entity cache and provide proper error handling.

Response format (200 OK):

[
  {
    "id": "powertrain__engine",
    "name": "engine",
    "namespace": "/powertrain/engine",
    "topic_count": 3
  }
]

Error format (404 Not Found):

{
  "error": "Area not found",
  "area_id": "nonexistent"
}

Implementation:

• Add REST handler for GET /areas/{area_id}/components
• Extract area_id from URL path parameter
• Filter components by area from entity cache
• Return 404 with error message if area doesn't exist
• Return array of components in that area if found

Motivation

This endpoint is essential for:

1. Area-scoped discovery: Enables clients to explore components within a specific domain (e.g., only powertrain components) without retrieving the entire system
2. Hierarchical navigation: Supports UI patterns where users first select an area, then drill down into components
3. Error handling patterns: Establishes proper REST error responses (404 for not found) that will be used consistently across all endpoints
4. Performance optimization: Reduces payload size when clients only need to inspect one area of the robot

Alternatives considered

N/A

Additional context

N/A

Acceptance Criteria:

• curl http://localhost:8080/areas/powertrain/components returns components in powertrain area
• curl http://localhost:8080/areas/nonexistent/components returns 404 with error message
• colcon test passes
• README documents endpoint with both success and error examples