Hello,
I’ve encountered a problem when using Task.async_stream/3 within tests where sandbox shared mode is enabled.

I have the following test:

defmodule MyApp.SharedModeTest do
  use MyApp.DataCase, async: false

  test "shared mode works" do
      MyApp.Repo.transaction(fn ->
        Task.async_stream(
          [1],
          fn id -> MyApp.Repo.get(MySchema, id) end,
          timeout: :infinity
        )
        |> Enum.to_list()
      end)
  end
end

DataCase content looks as follow:

  setup tags do
    :ok = Sandbox.checkout(MyApp.Repo)

    unless tags[:async] do
      Sandbox.mode(MyApp.Repo, {:shared, self()})
    end

    :ok
  end

It fails with the following exception:

** (EXIT from #PID<0.3878.0>) exited in: DBConnection.Holder.checkout(#PID<0.3879.0>, [log: #Function<13.86356856/1 in Ecto.Adapters.SQL.with_log/3>, source: "my_table", cast_params: [1], repo: MyApp.Repo, timeout: 15000, pool_size: 10, pool: DBConnection.Ownership, ownership_timeout: 600000, queue_target: 600000, queue_interval: 600000])
         ** (EXIT) shutdown: %DBConnection.ConnectionError{message: "client #PID<0.3878.0> timed out because it queued and checked out the connection for longer than 15000ms\n\nClient #PID<0.3878.0> is still using a connection from owner at location:\n\n    (elixir 1.17.1) lib/task/supervised.ex:292: Task.Supervised.stream_reduce/7\n    (elixir 1.17.1) lib/enum.ex:4423: Enum.reverse/1\n    (elixir 1.17.1) lib/enum.ex:3748: Enum.to_list/1\n    (ecto_sql 3.11.3) lib/ecto/adapters/sql.ex:1358: anonymous fn/3 in Ecto.Adapters.SQL.checkout_or_transaction/4\n    (db_connection 2.6.0) lib/db_connection.ex:1710: DBConnection.run_transaction/4\n    ...

Apparently, db connection isn’t shared with task process, therefore it tries to check it out, but fails because it is already checked out by test process.
If I remove transaction from the test, it works as expected:

defmodule MyApp.SharedModeTest do
  use MyApp.DataCase, async: false

  test "shared mode works" do
      Task.async_stream(
        [1],
        fn id -> MyApp.Repo.get(MySchema, id) end,
        timeout: :infinity
      )
      |> Enum.to_list()
  end
end

My perception is that transaction must not affect db connection sharing among processes.
Is it a bug, or I misunderstand how it must work?