Wind down spawned tasks when startup fails

[Jolah1]

Fixes #1009.

Node::start spawns background tasks — wallet sync, RGS gossip, pathfinding scores — before it can still fail, most visibly when resolving or binding the configured listening addresses. The error arm only called chain_source.stop(), so those tasks kept running behind a node that never came up.

This extracts the wind-down sequence out of Node::stop into a private Node::shutdown and runs it on any start_inner error.

Reviewer note — relaxed assertions. Because the wind-down now also runs after a partial startup, it can no longer assume every task exists:

• Both shutdown watch::Sender::send calls are allowed to find no receivers (previously log_error! + debug_assert!(false)).
• Runtime::wait_on_background_tasks' debug_assert!(tasks.len() > 0) and Runtime::wait_on_background_processor_task' debug_assert!(false, "Expected a background processing task") are dropped, with doc comments in their place explaining that the empty case is now reachable by design.

I considered keeping the assertions by threading an expect_running: bool through shutdown(), but that trades a real invariant for a parameter the caller can get wrong, and the assertions only ever restated "we got here via stop". Happy to go the other way if you'd rather keep them.

Testing

• tests/integration_tests_rust.rs::failed_start_winds_down_background_tasks — squats one of the node's listening addresses so bind fails after the wallet-sync, RGS and pathfinding-scores tasks have been spawned, asserts via a CollectingLogWriter that the full shutdown sequence ran, then frees the address and confirms the node starts and stops cleanly.
• src/runtime.rs::winding_down_without_spawned_tasks_is_a_noop — the runtime wind-down calls tolerate finding nothing to wait on.

This PR was written with the assistance of Claude Code.

[ldk-reviews-bot]

I've assigned @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[elnafateh]

The test covers only wallet sync and pathfinding scores but never RGS. Worth adding, or at least noting why it's left out, Overall good work though.

[Jolah1]

The test covers only wallet sync and pathfinding scores but never RGS. Worth adding, or at least noting why it's left out, Overall good work though.

Good catch, thanks! Looking into it turned up a second gap: the test had background sync disabled, so the wallet-sync task exited immediately and there was nothing left for the wind-down to stop. Fixed now: background sync stays enabled, and the test asserts that the wallet-sync task logged its stop line.