Test servers: a showcase config that broadcasts resource updates, for manual --load-subscribe fan-out runs #2964

Found in the v2.15.0 release smoke (#2960), while driving --load-subscribe (#2913 / PR #2927) by hand.

Problem

No showcase test-server config broadcasts a resource update to every subscribed session. Over Streamable HTTP, each session of the composable server is its own server instance, so update_resource notifies only the session that called it. A manual fan-out run against the stock fixture therefore looks like lost deliveries:

server-composable.js with subscriptions:true + numbered_resources + update_resource
$ … --method tools/call --tool-name update_resource --tool-arg uri=test://resource_1 \
    --tool-arg 'text=v{{seq}}' --load --load-requests 10 \
    --load-subscribe test://resource_1 --load-subscribers 5
Delivered: 10 of 50 … 40 missed across 4 sessions

The CLI is reporting this correctly. Against a broadcasting server built with onResourceSubscribe, the way the CLI's own test builds one, the same run delivered 200 of 200.

Ask

Add a showcase config, or a composable option, whose resource updates reach every subscribed session. Point the --load-subscribe docs at it so the feature can be tried by hand without writing a server.