Remove `nursery` parameter from `IListener.listen()` (breaking)

[yashksaini-coder]

Problem

IListener.listen(maddr, nursery) currently requires every caller to provide a trio.Nursery whose lifetime must outlive the listener. This leaks listener background-task ownership into callers, makes per-listener cleanup ambiguous, and conflicts with
transports (QUIC) that already manage their own background work.

Tracked in Discussion #726 (Issue 1).

Proposal

Drop the nursery parameter from IListener.listen(). Concrete listeners (TCP, WebSocket, QUIC, Circuit Relay v2) own their own background tasks internally via trio.lowlevel.spawn_system_task and are torn down via an explicit close() call.

Acceptance criteria

• IListener.listen(maddr) no longer accepts a nursery parameter.
• TCPListener, WebSocketListener, QUICListener, and CircuitV2Listener own their background task lifetimes internally.
• Swarm shutdown calls listener.close() explicitly on every listener.
• Breaking-change newsfragment shipped under newsfragments/<this-issue>.breaking.rst.
• Callers in tests/utils/factories.py and example code migrated.
• TCPListener correctly handles repeated listen() calls on the same instance (multi-bind contract) without leaking orphaned nurseries.

Implementing PR

#1308