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
 

agent/lib/proxmox_agent/reporter.ex

@@ -47,7 +47,7 @@ defmodule ProxmoxAgent.Reporter do
     payload = %{collected_at: DateTime.utc_now() |> DateTime.to_iso8601(), data: sample}
     :ok = push_metric(socket, "metric:fast", payload)
     Process.send_after(self(), :collect_fast, socket.assigns.cfg.fast_seconds * 1000)
-    {:ok, socket}
+    {:noreply, socket}
   end
 
   @impl Slipstream