fix: Sandbox owner checks in DB connection before exiting

The shared sandbox owner process exited without releasing its Postgrex
connection, causing intermittent "client exited" error logs on CI. The
owner now calls Sandbox.checkin before exiting, and on_exit waits for
the owner to finish before switching to manual mode.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/support/integration_case.ex

@@ -47,7 +47,8 @@ defmodule Parrhesia.IntegrationCase do
             send(test_pid, {:sandbox_ready, self()})
 
             receive do
-              :stop -> :ok
+              :stop ->
+                Sandbox.checkin(Repo)
             end
           end)
 
@@ -60,8 +61,16 @@ defmodule Parrhesia.IntegrationCase do
         Sandbox.mode(Repo, {:shared, owner})
 
         on_exit(fn ->
-          Sandbox.mode(Repo, :manual)
+          ref = Process.monitor(owner)
           send(owner, :stop)
+
+          receive do
+            {:DOWN, ^ref, :process, ^owner, _reason} -> :ok
+          after
+            5_000 -> :ok
+          end
+
+          Sandbox.mode(Repo, :manual)
         end)
     end