feat(opcua): raise PLC_COMMS_LOST fault on connection loss

[mfaferek93]

Raises a single component-scoped PLC_COMMS_LOST fault (severity ERROR, configurable) when the OPC UA connection stays down, so a controller going dark surfaces on /faults instead of only the x-plc-status connected boolean. Brings the opcua plugin to parity with the modbus/s7/ads bridges.

• Debounced: raised only after the connection is continuously down for comms_lost_debounce_ms (default 5000), so a brief blip during a normal reconnect does not flap a fault. Idempotent (raised once), cleared on the next successful reconnect.
• Component-scoped to the node map's root component; message carries the endpoint.
• Config knobs: comms_lost_fault_enabled (default true), comms_lost_debounce_ms (default 5000), comms_lost_severity (default ERROR); env OPCUA_COMMS_LOST_ENABLED / OPCUA_COMMS_LOST_DEBOUNCE_MS.
• The debounce gate is a pure static helper with unit tests (CommsLostShouldRaise).

Closes #496

[Copilot]

Pull request overview

Adds debounced “PLC communications lost” fault emission to the OPC-UA plugin so prolonged connection loss becomes visible via /faults, aligning behavior with other PLC bridges and issue #496’s expectations.

Changes:

• Introduces a debounced comms-lost raise/clear path in the OPC-UA poller (PLC_COMMS_LOST, component-scoped).
• Adds configuration knobs (YAML + env overrides) for enabling and tuning the comms-lost fault behavior and severity.
• Adds unit tests for the pure debounce gate helper and updates plugin README parameter documentation.

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_plugins/ros2_medkit_opcua/test/test_opcua_plugin.cpp	Adds unit tests for the comms-lost debounce decision helper.
src/ros2_medkit_plugins/ros2_medkit_opcua/src/opcua_poller.cpp	Raises/clears PLC_COMMS_LOST based on debounced connection-down duration.
src/ros2_medkit_plugins/ros2_medkit_opcua/src/opcua_plugin.cpp	Parses new comms-lost config knobs (JSON + env overrides).
src/ros2_medkit_plugins/ros2_medkit_opcua/README.md	Documents new comms-lost YAML parameters in the gateway parameter table.
src/ros2_medkit_plugins/ros2_medkit_opcua/include/ros2_medkit_opcua/opcua_poller.hpp	Adds comms-lost config fields and poller state for debouncing/idempotence.

[bburda]

Additional notes outside the diff:

• ros2_medkit_opcua/design/index.rst documents the PLC condition/event to fault flow but not PLC_COMMS_LOST, which is different in kind (raised by the poll thread on connection loss, not sourced from a PLC condition node). A short note there would keep the design doc in step with the README.
• Only the pure comms_lost_should_raise gate is unit-tested. The clear path (emit_comms_lost(false) plus resetting comms_lost_raised_ and comms_down_since_ on reconnect) has no coverage and is the more state-heavy half. Extracting a pure reconnect-transition helper would let it be tested the same way as the raise gate.