fix(cluster): report a dead yamux session instead of swallowing it

Matching yamux errors with errors.Is was too broad. Session.close hands every
live stream ErrStreamReset wrapped around whatever killed the connection, so a
keepalive timeout, a broken TCP connection or a peer that simply vanished all
matched, and a relayed request that died reported a clean ending. Nothing
upstream would have retried or logged it.

Match the plain sentinels by identity, since only identity separates a stream
that was reset from the wrapped form that means the session died. Treat a
StreamError as a per-stream reset, and a GoAwayError as normal only when it
carries the no-error code, read off ErrRemoteGoAway because the constant is
unexported. ErrSessionShutdown needs no entry of its own; it is a GoAwayError
with that code.

Order matters as much as the matching: session death wraps its cause, which is
routinely io.EOF or a closed socket, so the mux checks run before the generic
endings. Reversing them alone puts a vanished peer back to nil.

Assisted-by: Claude Opus 5 [claude-code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f