Silence datagram cleanup after event-loop teardown

[ampledata]

Summary

• make DatagramStream finalizer best-effort when asyncio loop teardown already happened
• close multicast regression transports inside their owning event loop
• add a direct destructor regression
• document the cleanup fix in the pending 7.5.1 notes

Validation

• 240 passed on Python 3.13
• no PytestUnraisableExceptionWarning from DatagramStream
• git diff --check