test(memory): stop the loop-restart test's stale task being logged later

[allen0099]

Closes #295

Problem

test_cleanup_restarts_on_a_new_loop_after_the_old_one_closed leaves a MemoryBackend cleanup task pending on a closed loop on purpose (the #181 scenario). When the garbage collector freed that task, asyncio logged Task was destroyed but it is pending! during an unrelated later test. asyncio logs this instead of warning, so filterwarnings = ["error"] never caught it.

Changes

• The test releases its stale task quietly. A task on a closed loop can't be cancelled, because cancelling schedules a callback on that loop and raises. So after asserting the MemoryBackend: cleanup task lifecycle across event loops #181 behaviour, and that the stale task is still pending, the test calls the public loop.set_exception_handler() on the closed loop. The handler drops only that one report and passes anything else to default_exception_handler. It relies on no private attribute.
• A stray pending task now fails the run. tests/conftest.py adds a handler to the asyncio logger that records the message and the test running when it fired. pytest_sessionfinish calls gc.collect() so that a task still waiting to be collected reports before the check. Any record then sets the exit status to failed, and a terminal-summary section prints the message and says to rerun with PYTHONASYNCIODEBUG=1 to find where the task was created.
• Every MemoryBackend a test builds is closed on the test's loop. The guard exposed 135 more leaked cleanup tasks in the lowest tox env. pytest-asyncio 0.26 (our floor) closes each test's loop without cancelling pending tasks. 1.x runs the test in an asyncio.Runner, which cancels them, so the leak didn't show there.
  • setup_default_backend is now async and awaits aclose() while the loop is still open. Its sync stop_cleanup() used to run after the loop had already closed.
  • A new autouse close_memory_backends fixture records every MemoryBackend built during a test, as close_network_clients does for network clients. That includes the library's fallback backend. The fixture closes each one at teardown.

Tests only. No library change and no changelog fragment, since nothing user-facing changes.

Verification

• uv run pytest -p no:logging -s 2>&1 | grep "Task was destroyed": one match before, none after (also none with PYTHONASYNCIODEBUG=1).
• tox -e lowest and the latest-dependency suite both pass with the guard active.
• With the fix in the test reverted, the suite fails on both dependency sets with the new summary section, and the single test fails on its own too.
• ruff, pre-commit and the full suite with coverage pass.