PLC_COMMS_LOST on a plugin component carries no freeze frame

[bburda]

Summary

docs/tutorials/snapshots.rst ("Zero-Config Entity Freeze-Frames", "Plugin-backed entities") says a plugin-backed entity that loses its link gets a freeze frame with the plugin's last known values and connected: false.

On an OPC UA gateway that lost its controller, the fault carries no freeze frame. GET /api/v1/components/openplc_a/faults/PLC_COMMS_LOST serves one rosbag entry and nothing else, on the confirm edge and after two gateway restarts with the fault standing:

"snapshots": [
  {"type": "rosbag", "name": "rosbag_PLC_COMMS_LOST",
   "bulk_data_uri": "/components/openplc_a/bulk-data/rosbags/PLC_COMMS_LOST",
   "duration_sec": 6.0, "format": "sqlite3", "size_bytes": 33575}
]

Cause: PLC_COMMS_LOST is reported by the component openplc_a, and a component serves no data.

• GET /api/v1/components/openplc_a/data answers 404, No data provider for plugin entity 'openplc_a'
• x-plc-data is registered on apps only. GET /api/v1/components/openplc_a/x-plc-data answers 404.
• The capture's route fallback builds /apps/openplc_a/x-plc-data, which answers 400, Entity 'openplc_a' is a Component, not a App.

Those are the two sources the entity freeze-frame capture has. The hosted app load_process does serve data during the outage (GET /api/v1/apps/load_process/x-plc-data answers with the last polled values and connected: false), and its own faults get frames. The values exist one level below the entity the fault landed on.

Proposed solution

1. When the reporting entity is a component with no data provider, capture from the entities it hosts. GET /api/v1/components/openplc_a/faults already lists the hosted app's faults with their source_id, so the relation is available at capture time.
2. Have the plugin export a component-level data provider with the last known values and connected: false.
3. Or state in snapshots.rst that plugin freeze frames are captured for app entities only.

Additional context

Reproduced on ghcr.io/selfpatch/ros2_medkit-jazzy at f873448 (0.6.0) and on a 0.7.0 build carrying #660. ROS 2 Jazzy, Ubuntu 24.04, OPC UA plugin on an OpenPLC-style controller with a node map, link cut by stopping the controller.

#564 stopped the capture refusing a payload with connected: false, but this fault lands on an entity that serves no payload. #562 added the startup catch-up, which the restarts above exercise. #515 (declared collections) and #663 (points through /apps/{id}/data) change where the points are served and give the component nothing to read.