Implement `GET /apps/{app-id}/is-located-on` endpoint

[bburda]

Summary

Add a new discovery endpoint that returns the Component on which a given App runs. This is the reverse of the existing GET /components/{id}/hosts endpoint (which lists Apps hosted by a Component).

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Proposed solution

GET /api/v1/apps/{app-id}/is-located-on

Returns the parent Component that hosts this App.

Path parameters:

Parameter	Type	Required	Description
app-id	string	Yes	App identifier (alphanumeric, underscores, hyphens, max 256 chars)

Request body: None

Response 200 OK:

{
  "items": [
    {
      "id": "engine_ecu",
      "name": "Engine ECU",
      "href": "/api/v1/components/engine_ecu"
    }
  ],
  "x-medkit": {
    "total_count": 1
  },
  "_links": {
    "self": "/api/v1/apps/{app-id}/is-located-on",
    "app": "/api/v1/apps/{app-id}"
  }
}

The items array will contain exactly 0 or 1 component — an App runs on at most one Component. If the App has no associated Component (e.g., standalone manifest-defined App), return an empty array.

Response 400 Bad Request:

Returned when the app-id format is invalid (empty, too long, contains illegal characters).

{
  "error_code": "invalid-parameter",
  "message": "Invalid app ID: contains illegal characters",
  "parameters": {
    "app_id": "bad id!"
  }
}

Response 404 Not Found:

Returned when no App with the given ID exists in any collection.

{
  "error_code": "entity-not-found",
  "message": "App not found",
  "parameters": {
    "app_id": "nonexistent_app"
  }
}

---

Additional context

Existing patterns to follow

• Handler pattern: Use HandlerContext::validate_entity_id() for ID validation, then look up the App via DiscoveryManager::get_app(app_id). Follow the same structure as handle_app_depends_on in discovery_handlers.cpp.
• Data source: The App struct already has a component_id field that holds the parent component ID. If non-empty, look up the Component to build the EntityReference.
• Response envelope: Use the standard { "items": [...], "_links": {...} } format used by all other relationship endpoints.

Route registration

Register in rest_server.cpp::setup_routes():

srv->Get((api_path("/apps") + R"(/([^/]+)/is-located-on$)"), handler);

Place this before the generic GET /apps/{id} route to ensure correct matching.

Handler method

Add to DiscoveryHandlers:

void handle_app_is_located_on(const httplib::Request& req, httplib::Response& res);

Tests

• Unit test: verify correct component returned for app with known component_id
• Unit test: verify empty items array when app has no component
• Unit test: 404 for nonexistent app
• Unit test: 400 for invalid app ID format
• Integration test: launch demo nodes, query the endpoint, verify response