Implement GET /components endpoint to list all discovered components across all areas

[mfaferek93]

Feature request / General issue

Implement GET /components endpoint to list all discovered components across all areas in the ROS 2 Medkit Gateway.

Proposal

Add a REST API endpoint at GET /components that returns a comprehensive list of all discovered components in the system. The endpoint should retrieve components from the
entity cache and return them with metadata including:

• Component ID (formatted as area__name)
• Component name
• Parent area
• Namespace path
• Topic count
• Service count

Response format:

 [
    {
      "id": "powertrain__engine",
      "name": "engine",
      "area": "powertrain",
      "namespace": "/powertrain/engine",
      "topic_count": 3,
      "service_count": 1
    },
    ...
  ]

Motivation

This endpoint is essential for:

1. System-wide visibility: Provides a single API call to discover all components in the robot system
2. Remote diagnostics: Enables remote tools to quickly understand the complete system topology
3. Foundation for component operations: Establishes the component listing capability needed before implementing component data read/write endpoints

Alternatives considered

N/A

Additional context

N/A

Acceptance Criteria:

• curl http://localhost:8080/components returns array of components
• Response includes all required fields (id, name, area, namespace)
• colcon test passes all tests
• README documents the endpoint with curl example