feat(stream): add Control::accept_with_buffer

[nikicat]

Description

Control::accept creates the channel behind IncomingStreams with a capacity of zero, and the
connection handler hands each fully negotiated inbound stream over with try_send from within
the swarm's poll. If a second stream for the same protocol arrives before the consumer has taken
the previous one, it is dropped even though negotiation already succeeded: the remote sees a
successful open_stream (and possibly a successful write) for data that is never delivered.
This happens as soon as several peers open a stream for the same protocol at about the same time
(one-stream-per-message protocols hit it routinely).

This PR adds Control::accept_with_buffer(protocol, n), which queues up to n negotiated streams.
accept keeps its current behaviour (it is accept_with_buffer(protocol, 0)), so the change is
non-breaking and bumps libp2p-stream to 0.5.0-alpha.1. Streams dropped because the buffer is
full are now logged at warn instead of debug, since the remote cannot tell they were lost.

The new test opens 16 streams while nobody polls IncomingStreams and expects all 16 to be
delivered; it fails with a buffer of 0.

AI Assistance Disclosure

Tools used: Claude Code

Attestation:

• I have read every line of this diff, understand what it does, and can explain it in review.

Notes & open questions

• An alternative would be to make accept itself buffered with a default capacity, or to put the
  capacity into a Behaviour config. I kept accept unchanged to avoid changing behaviour for
  existing users; happy to switch if maintainers prefer one of those.
• Even with a buffer, a burst larger than n is still dropped, because the handler cannot wait
  for the consumer from within poll. The warn log makes that visible.

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

🤖 Generated with Claude Code