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

[eclipse0922]

Pull Request

Summary

• add GET /apps/{app-id}/is-located-on discovery route and handler
• return the host component as a 0-or-1 items response, including x-medkit.missing=true when an unresolved host reference is encountered
• add unit and integration coverage for success, empty result, invalid ID, unknown app, and endpoint registration

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Issue

Link the related issue (required):

• closes Implement GET /apps/{app-id}/is-located-on endpoint #195

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Type

• Bug fix
• New feature or tests
• Breaking change
• Documentation only

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Testing

• Built in the devcontainer with source /opt/ros/jazzy/setup.bash && colcon build --packages-up-to ros2_medkit_integration_tests --build-base build-dev195 --install-base install-dev195
• Ran source /opt/ros/jazzy/setup.bash && source install-dev195/setup.bash && colcon test --packages-select ros2_medkit_gateway --build-base build-dev195 --install-base install-dev195
• Ran colcon test-result --verbose --test-result-base build-dev195/ros2_medkit_gateway (2119 tests, 0 errors, 0 failures, 244 skipped)
• Ran source /opt/ros/jazzy/setup.bash && source install-dev195/setup.bash && colcon test --packages-select ros2_medkit_integration_tests --build-base build-dev195 --install-base install-dev195 --ctest-args -R "test_health|test_hateoas" --output-on-failure
• Ran colcon test-result --verbose --test-result-base build-dev195/ros2_medkit_integration_tests (27 tests, 0 errors, 0 failures, 0 skipped)

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Checklist

• Breaking changes are clearly described (and announced in docs / changelog if needed)
• Tests were added or updated if needed
• Docs were updated if behavior or public API changed

[Copilot]

Pull request overview

Adds a new discovery relationship endpoint to the ROS 2 Medkit Gateway REST API: GET /apps/{app-id}/is-located-on, allowing clients to resolve an App’s host Component (the inverse of /components/{id}/hosts).

Changes:

• Register GET /apps/{app-id}/is-located-on in RESTServer::setup_routes().
• Implement DiscoveryHandlers::handle_app_is_located_on() to return a 0-or-1 items list with _links and x-medkit.total_count.
• Add/extend unit + gateway HTTP tests for success, empty result, invalid ID, and unknown app cases.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/ros2_medkit_gateway/src/http/rest_server.cpp	Registers the new /is-located-on route for apps.
src/ros2_medkit_gateway/src/http/handlers/discovery_handlers.cpp	Implements the new handler logic and response envelope.
src/ros2_medkit_gateway/include/ros2_medkit_gateway/http/handlers/discovery_handlers.hpp	Declares the new handler method.
src/ros2_medkit_gateway/test/test_discovery_handlers.cpp	Adds unit tests for the new relationship endpoint and adjusts seeded manifest expectations.
src/ros2_medkit_gateway/test/test_gateway_node.cpp	Adds an HTTP-level test and a manifest fixture for the new endpoint.

[bburda]

Thanks for this PR! LGTM!