Function build_graph 

pub(crate) fn build_graph(detail: &DiagnosticsRoomDetail) -> Value

Build the Grafana node graph payload for a single room detail response.

The returned JSON has two top-level arrays:

• nodes contains the room, live sessions, publication sources and any missing source placeholders needed by subscription edges.
• edges contains membership edges, publication upload edges and download subscription edges.

This function is compatibility-shaped around Grafana’s node graph field names. Domain code should keep using DiagnosticsRoomDetail.