feat(node): support unavailable node handling

[penggin]

Summary

Add on_node_unavailable that is dispatched when Node.available become False
Modify NodePool.nodes to return only available nodes, preventing players from attempting to connect to unavailable nodes.
Fix doesn't wait for ready Node.connect when reconnecting to the node.

Checklist

• If code changes were made then they have been tested.
• I have run task lint to format code and my changes.
• I have run task pyright and fixed the relevant issues.

[ooliver1]

Could the event be documented?

[penggin]

Sorry for late response!
While making these changes, I found 'node_ready' event, and it's also not documented.
Will I add this, too?

[ooliver1]

Yeah sure!