fix(kad): enforce a timeout for inbound substreams

[teor2345]

Description

In the kad substream handler, outbound substreams have a 10s timeout, but inbound substreams don't have any timeout.

This results in large numbers of warnings under specific heavy load conditions, which we have encountered in subspace:

2025-04-08T06:24:27.293722Z WARN Consensus: libp2p_kad::handler: New inbound substream to peer exceeds inbound substream limit. No older substream waiting to be reused. Dropping new substream. peer=PeerId("12D3KooWN6kFp2Ev181UGq3BUDfk1jfjaNu6sDTqxCZUBpmp8kRQ")

Fixes #5981

Notes & open questions

Should the substream be closed on a timeout?
The existing code doesn't close them on (most) substream errors, so this PR handles timeouts the same way.

I have been unable to replicate the specific conditions leading to this bug in a test or my local subspace node, but we've confirmed the fix works on multiple nodes in the subspace network.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates