feat: add Reachability Center and Why Drawer components with tests

- Implemented ReachabilityCenterComponent for displaying asset reachability status with summary and filtering options.
- Added ReachabilityWhyDrawerComponent to show detailed reachability evidence and call paths.
- Created unit tests for both components to ensure functionality and correctness.
- Updated accessibility test results for the new components.

docs/api/gateway/samples/exception-events.ndjson

@@ -0,0 +1,3 @@
+{"type":"exception.created","tenantId":"tenant-default","exceptionId":"exc-001","timestamp":"2025-12-10T00:00:00Z","actor":"user:demo","traceId":"trace-evt-1","metadata":{"message":"Draft created"}}
+{"type":"exception.status_changed","tenantId":"tenant-default","exceptionId":"exc-001","timestamp":"2025-12-10T00:05:00Z","actor":"user:approver","previousStatus":"pending_review","newStatus":"approved","traceId":"trace-evt-2","metadata":{"comment":"Approved for demo tenant."}}
+