A long-lived backend (Desktop's tui_gateway) runs MCP discovery once. Every
agent build re-enters start_background_mcp_discovery, but that returned as
soon as any server was connected, so a server added with `hermes mcp add`
after startup never reached a new session; only /reload-mcp or a restart
picked it up.
Re-entry now also runs discovery when an enabled configured server is not
live and its connect cooldown has lapsed. Discovery is additive, so live
servers and open sessions are untouched; the new session gets the tools.
The pending-server computation is factored out of
reconcile_mcp_servers_with_config so both callers share it.
Refs #76954
a052836559