test: wait on observable state instead of blind sleeps

Replace the fixed Process.sleep waits in the import, members-PDF and
field-visibility tests with event-based / bounded-poll waits on the
observable condition, removing a known flakiness vector.

test/mv/membership/members_pdf_test.exs

@@ -274,17 +274,38 @@ defmodule Mv.Membership.MembersPDFTest do
 
       assert {:ok, _pdf_binary} = result
 
-      # Wait a bit for cleanup (async cleanup might take a moment)
-      Process.sleep(100)
-
-      # Count temp directories after
-      after_count =
+      count_export_dirs = fn ->
         temp_base
         |> File.ls!()
         |> Enum.count(fn name -> String.starts_with?(name, "mv_pdf_export_") end)
+      end
+
+      # Poll the observable cleanup condition (temp-dir count returns to the baseline)
+      # with a bounded deadline instead of a fixed sleep, so the test waits no longer
+      # than the cleanup actually needs and still fails if cleanup never runs.
+      after_count = poll_until_cleaned(count_export_dirs, before_count, 100)
 
       # Should have same or fewer temp dirs (cleanup should have run)
       assert after_count <= before_count + 1
     end
   end
+
+  # Bounded poll: returns the export-dir count once it drops back to the baseline
+  # (cleanup done), or the last observed count when the attempt budget is exhausted
+  # (so the caller's assertion reports the real state on a genuine cleanup stall).
+  defp poll_until_cleaned(count_fun, baseline, attempts_left) do
+    current = count_fun.()
+
+    cond do
+      current <= baseline ->
+        current
+
+      attempts_left <= 0 ->
+        current
+
+      true ->
+        Process.sleep(10)
+        poll_until_cleaned(count_fun, baseline, attempts_left - 1)
+    end
+  end
 end