fix(gateway): consistent parameter-config error surface (list vs get) and drop dead classify fallback

[mfaferek93]

Two follow-ups from the #540 review, both in the parameter-error path of config_handlers.cpp.

#541 - consistent list vs get error surface

GET /{entity}/configurations silently dropped a down backing node and returned a plain 200, while GET /{entity}/configurations/{param} returns 503 for the same outage. The list side now flags the response partial and names the down nodes in unavailable_nodes (mirroring the existing peer fan-out partial/failed_peers convention) instead of returning a shrunken list with no signal. It stays 200 (partial success is legitimate for a collection); the all-nodes-down case already returns 503. Documented in the REST API guide.

#542 - drop dead string-matching classify fallback

Every failure path on the shipped Ros2ParameterTransport sets a structured error_code, so the error_code == NONE string-matching fallback was dead - and wrong if ever reached (its substrings never matched the real "did not respond" / "not currently set" messages, so failures fell through to 400 invalid-request). The fallback is removed; the classifier is extracted into its own translation unit and a NONE-coded failure now maps to 500 internal-error, never a 400.

Tests

• New test_parameter_error_classification unit test pins the structured mapping and asserts a NONE failure cannot reach a 400 misclassification.
• test_parameter_error_precedence gains handler-level cases: one backing node down surfaces partial + unavailable_nodes in both node orders, with a control asserting no partial flag when every node is reachable.
• colcon build + full gateway unit suite green in the jazzy image

Closes #541
Closes #542

[Copilot]

Pull request overview

This PR improves the gateway’s parameter-configuration error handling by (1) making GET /{entity}/configurations explicitly surface partial results when some backing ROS 2 nodes are unreachable, and (2) extracting/removing the dead legacy string-matching parameter error classifier in favor of a structured ParameterErrorCode mapping. It also adds unit/handler tests and updates REST API docs to describe the new partial-response metadata.

Changes:

• Surface partial list results via x-medkit.partial + x-medkit.unavailable_nodes for GET /{entity}/configurations when some local backing nodes fail.
• Extract structured parameter error classification into parameter_error_classification.{hpp,cpp} and remove the dead string-matching fallback.
• Add new tests to pin error classification and list-vs-get behavior; update REST API documentation accordingly.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/ros2_medkit_gateway/test/test_parameter_error_precedence.cpp	Adds handler-level tests for list endpoint partial signaling and backing-node ordering.
src/ros2_medkit_gateway/test/test_parameter_error_classification.cpp	New unit test pinning ParameterErrorCode → HTTP/SOVD error mapping, including NONE regression.
src/ros2_medkit_gateway/src/http/parameter_error_classification.cpp	Implements structured error-code classification in a dedicated translation unit.
src/ros2_medkit_gateway/src/http/handlers/config_handlers.cpp	Uses extracted classifier and adds unavailable_nodes/partial emission in list path.
src/ros2_medkit_gateway/include/ros2_medkit_gateway/dto/config.hpp	Extends ConfigListXMedkit DTO with unavailable_nodes.
src/ros2_medkit_gateway/include/ros2_medkit_gateway/core/http/parameter_error_classification.hpp	Introduces public classifier API + DTO for classification result.
src/ros2_medkit_gateway/CMakeLists.txt	Builds new classifier TU and adds the new unit test target.
docs/api/rest.rst	Documents the new local-backing-node partial response fields for configuration listing.