feat(gateway): expose entity context on /faults/stream SSE events

[bburda]

Pull Request

Summary

GET /api/v1/faults/stream event payloads now carry an optional x-medkit SOVD payload-extension object with entity_type and entity_id fields when the gateway can resolve the fault's first reporting_sources entry back to a SOVD entity. Consumers can then hit /{entity_type}/{entity_id}/bulk-data/rosbags/{fault_code} directly, replacing the O(N) HEAD-probe workaround that external integrations need today.

Nested under x-medkit per the SOVD payload-extension convention (matches existing x-medkit.aggregation_level, x-medkit.phase, etc.). Flat x-medkit-* names are reserved for endpoint paths (/x-medkit-graph) and error codes, not payload fields.

Resolution mirrors the existing TriggerFaultSubscriber / LogManager pattern:

• Manifest / hybrid mode: cache's node-to-app linking index (with and without leading /).
• Runtime-only fallback: ROS FQN's last segment, only emitted when an App with that id actually exists in the cache - we never point consumers at a 404.
• No match: x-medkit object omitted entirely (backward-compatible for existing SOVD consumers).

Resolution is snapshotted in on_fault_event and stored alongside the buffered event (new QueuedEvent struct), so a discovery refresh between enqueue and stream-out cannot retroactively flip the entity reported to consumers, and the format path stays lock-free wrt the entity cache.

Example payload:

{
  "event_type": "fault_confirmed",
  "fault": {"fault_code": "MOTOR_OVERHEAT", "...": "..."},
  "timestamp": 1735830000.123,
  "x-medkit": {"entity_type": "apps", "entity_id": "motor_controller"}
}

---

Issue

• closes Global /faults/stream SSE events lack entity context for downstream consumers #380

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Type

• Bug fix
• New feature or tests
• Breaking change
• Documentation only

---

Testing

• 4 new unit tests in test_sse_fault_handler.cpp covering: manifest-app match, runtime-fallback match, no-match (entity absent from cache), and empty reporting_sources.
• Full SSE fault handler suite (15 tests) green.
• Full gateway unit suite (2213 tests, 0 failures) green when run serially.
• clang-format / clang-tidy / ament-copyright clean on touched files.

---

Checklist

• Breaking changes are clearly described (and announced in docs / changelog if needed)
• Tests were added or updated if needed
• Docs were updated if behavior or public API changed

[Copilot]

Pull request overview

Adds an optional x-medkit SOVD payload-extension object (entity_type, entity_id) to events streamed from GET /api/v1/faults/stream, letting consumers jump directly to /{entity_type}/{entity_id}/bulk-data/rosbags/{fault_code} instead of HEAD-probing entities. Resolution is snapshotted at fault arrival time so a discovery refresh cannot retroactively change the reported entity, and the lookup mirrors the node_to_app mapping plus runtime-fallback heuristics already used by gateway_node / ros2_runtime_introspection.

Changes:

• Introduce QueuedEvent (id + event + resolved EntityContext) replacing the pair<id, event> in the SSE buffer, and resolve entity context before the queue lock in on_fault_event.
• Implement SSEFaultHandler::resolve_entity_context: manifest/hybrid via cache.resolve_node_to_app (with and without leading slash), runtime fallback to FQN last segment, and collision-disambiguated <ns_prefix>_<name> — only when the App exists in the cache.
• Document the new extension in the package README, top-level REST docs, and changelog; add 7 unit tests covering snapshot semantics, manifest/runtime/collision/no-leading-slash variants, and the omit cases.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/ros2_medkit_gateway/include/ros2_medkit_gateway/http/handlers/sse_fault_handler.hpp	Adds EntityContext/QueuedEvent types, new resolve_entity_context() declaration, and switches the event deque element type.
src/ros2_medkit_gateway/src/http/handlers/sse_fault_handler.cpp	Snapshots entity context at enqueue, emits x-medkit in format_sse_event, implements manifest + runtime fallback resolution with has_app validation.
src/ros2_medkit_gateway/test/test_sse_fault_handler.cpp	Adds 7 GTests covering match/no-match, runtime fallback, manifest mapping (with/without leading slash), collision-disambiguated app, snapshot-at-enqueue, and empty reporting_sources.
src/ros2_medkit_gateway/README.md	Documents the new x-medkit payload extension and updated example payload for the SSE stream endpoint.
src/ros2_medkit_gateway/CHANGELOG.rst	Adds the Unreleased feature entry referencing #380.
docs/api/rest.rst	Updates the global SOVD extensions section with the new field on /faults/stream.