fix(agent): jason-safe error entries + correct handle_info return

Errors produced by Collectors.Host were keyword tuples {:tag, msg}, which
Jason cannot encode — metric push crashed the channel. Convert them to
plain maps with :tag and :message fields.

Reporter.handle_info/2 returned {:ok, socket}, which Slipstream rejects
(GenServer-style {:noreply, socket} is the only valid return for that
callback, unlike handle_connect/handle_join/handle_disconnect).

agent/lib/proxmox_agent/collectors/host.ex

@@ -46,7 +46,7 @@ defmodule ProxmoxAgent.Collectors.Host do
     try do
       {fun.(), nil}
     rescue
-      e -> {fallback, {tag, Exception.message(e)}}
+      e -> {fallback, %{tag: Atom.to_string(tag), message: Exception.message(e)}}
     end
   end