DynamicSupervisor holding one live SessionServer per external id. Crash-isolated:
killing one server never affects a sibling. start_or_get/2 is the sole entry point —
it starts a fresh server or hands back the pid of one already registered under the same
external id, resolving the race where two callers both call it concurrently for the same id.
Summary
Functions
Start (or fetch, if a race lost) the SessionServer for external_id.
Functions
@spec start_or_get(String.t(), ReqManagedAgents.Host.Config.t()) :: {:ok, pid()} | {:error, term()}
Start (or fetch, if a race lost) the SessionServer for external_id.