feat(cluster): accept authenticated peer links on /api/cluster/peer

Upgrades to a WebSocket, wraps it as a yamux server session and hands it to
the caller. Rejects before upgrading so an unauthenticated dial sees a 401
rather than a WebSocket error, which is what the route-coverage test asserts.

The adapter keeps the reader of a partially consumed message across Read
calls. yamux reads through a 4 KiB bufio.Reader, so a small-payload test
cannot see a dropped message tail; the framing specs drive the adapter
directly with buffers smaller than the message.

An empty configured token authorizes nobody here, unlike the worker file
transfer server's check: this route is registered in every deployment, so
failing open would publish an unauthenticated mux.

Assisted-by: Claude Opus 5 [claude-code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
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