Test coverage gaps in merge pipeline and discovery

[bburda]

Summary

The merge pipeline has strong test coverage (1117-line test file) but several gaps were identified during self-review.

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Proposed solution (optional)

1. Area-specific field group merge tests

All merge tests use Component or App entities. Area field merge (IDENTITY, HIERARCHY, METADATA) is untested.

File: src/ros2_medkit_gateway/test/test_merge_pipeline.cpp

2. PluginLayer entity ID validation tests

PluginLayer::discover() calls validate_entities() which drops entities with invalid IDs. No tests exist for empty, oversized (>256 chars), or special-character IDs.

File: src/ros2_medkit_gateway/src/discovery/layers/plugin_layer.cpp:26-45

3. handle_health discovery block unit tests

Unit tests only cover nullptr GatewayNode path. The discovery info JSON construction (mode, strategy, pipeline report, linking stats) is never unit-tested.

File: src/ros2_medkit_gateway/test/test_health_handlers.cpp

4. Gap-fill positive assertion

test_discovery_gap_fill.test.py sets allow_heuristic_apps: true but only checks that areas/components are absent. If gap-fill stopped producing apps, tests would still pass.

File: src/ros2_medkit_integration_tests/test/features/test_discovery_gap_fill.test.py

5. merge_bool ENRICHMENT vs ENRICHMENT edge case

No test where two ENRICHMENT layers both provide is_online with conflicting values. The OR semantics should be explicitly tested.

File: src/ros2_medkit_gateway/test/test_merge_pipeline.cpp

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Additional context (optional)

Found during self-review of PR #258. These can all be addressed in a single PR.