feat(opcua): route alarms to distinct faults by message substring

[mfaferek93]

Adds a match_message field to event_alarms[].mappings - a case-sensitive substring match on the event Message, AND-combined with the existing condition_name / source_node / event_type matchers. This lets one OPC UA source route alarms that share an EventType and SourceNode (e.g. Program_Alarms from one Siemens S7-1500 FB) to distinct SOVD faults by their text.

• AlarmMapping gains match_message; resolve_alarm takes the event Message and skips a mapping whose non-empty match_message is not a substring of it. condition_name / source_node / event_type stay equality matches.
• Parsed from mappings[].match_message; the poller passes the live event Message (and the read-path snapshot message).
• README example + unit test (MatchByMessageSubstring) covering a match, the catch-all fallback, and an empty message.

Closes #505

[Copilot]

Pull request overview

Adds OPC UA alarm-routing support to disambiguate alarms that share condition_name / source_node / event_type by additionally matching on a case-sensitive substring of the event Message, enabling distinct SOVD fault codes for same-source PLC alarms (per #505).

Changes:

• Extend AlarmMapping with match_message and update NodeMap::resolve_alarm(...) to AND-combine substring matching with existing equality matchers.
• Plumb the live/snapshot event message through OpcuaPoller into alarm resolution.
• Update plugin documentation and add unit coverage for message-substring routing and fallback behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/ros2_medkit_plugins/ros2_medkit_opcua/test/test_node_map.cpp	Updates existing resolve-alarm tests for new signature and adds MatchByMessageSubstring coverage.
src/ros2_medkit_plugins/ros2_medkit_opcua/src/opcua_poller.cpp	Passes event/snapshot message into NodeMap::resolve_alarm for runtime + read-replay paths.
src/ros2_medkit_plugins/ros2_medkit_opcua/src/node_map.cpp	Parses match_message from YAML and applies substring routing in resolve_alarm.
src/ros2_medkit_plugins/ros2_medkit_opcua/README.md	Documents match_message behavior and clarifies matcher semantics.
src/ros2_medkit_plugins/ros2_medkit_opcua/include/ros2_medkit_opcua/node_map.hpp	Adds match_message to AlarmMapping and updates the resolve-alarm API surface.

[bburda]

Additional findings outside the diff (both in ros2_medkit_opcua/README.md):

• The event_alarms example uses PLC_OVERPRESSURE twice: once on the simple-form source (entity_id: tank_process) and once in the first mapping of the line_1 source. The loader rejects a fault code reused across sources, which the comment just above this example states, so this example fails to load if copied as-is. It predates this PR, but since you are editing this block, renaming one PLC_OVERPRESSURE would make the documented example loadable.
• The multi-alarm form comment still says "the first whose non-empty match fields all equal the observed event wins". With match_message being a substring match, this is no longer accurate. You updated the precedence paragraph below (equal to match), so this inline comment can get the same fix.