fix(opcua): accept severity as an alias in event_alarms

[mfaferek93]

Fixes event_alarms silently dropping a severity: key.

• node_map.cpp: the event_alarms loader now accepts severity: as an alias for severity_override: (source-level and per-mapping), routed through validate_severity like the threshold alarm: path, and warns on unknown keys in the block.
• The final severity resolution (explicit configured severity wins over the live event Severity; otherwise the Severity band mapping) is extracted into a small testable helper.
• Tests: an event_alarms entry with severity: ERROR resolves to ERROR; the helper covers override-wins and the band boundaries.

Closes #500.

[Copilot]

Pull request overview

This PR fixes OPC UA event_alarms configuration parsing so a severity: key is no longer silently ignored, aligning behavior with the threshold alarm: block and the acceptance criteria from issue #500. It also extracts the event-alarm severity resolution into a small helper to make the precedence/band logic unit-testable.

Changes:

• Accept severity: as an alias for severity_override: in event_alarms (source-level and per-mapping), validated via the existing severity validation path, with severity_override taking precedence when both are present.
• Warn on unknown keys in event_alarms entries and their mappings to prevent future typos from being silently dropped.
• Extract live-severity band mapping + override precedence into OpcuaPlugin::map_severity(...) and add unit/regression tests.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/ros2_medkit_plugins/ros2_medkit_opcua/src/node_map.cpp	Parses severity: as an alias for severity_override: in event_alarms and warns on unknown keys.
src/ros2_medkit_plugins/ros2_medkit_opcua/src/opcua_plugin.cpp	Extracts event-alarm severity resolution into map_severity() and uses it in on_event_alarm().
src/ros2_medkit_plugins/ros2_medkit_opcua/include/ros2_medkit_opcua/opcua_plugin.hpp	Declares the new map_severity() helper for unit testing.
src/ros2_medkit_plugins/ros2_medkit_opcua/test/test_node_map.cpp	Adds regression tests ensuring severity: sets the override and precedence behaves as intended.
src/ros2_medkit_plugins/ros2_medkit_opcua/test/test_opcua_plugin.cpp	Adds unit tests covering override-wins behavior and live-severity band boundary mapping.

[bburda]

Additional notes outside the inline comments:

• Behaviour change for existing configs: routing severity_override through validate_severity means a config with a non-canonical string (e.g. severity_override: high) no longer reaches the fault verbatim. It becomes ERROR with a warning. This matches the threshold alarm: path and is an improvement, but it changes the emitted severity for such configs. Worth a line in the PR description or CHANGELOG.
• README.md event_alarms section documents severity_override but not the new severity alias. A one-line note that event_alarms also accepts severity: would match the fix.