fix(server): report a yamux stream that could not be opened

[otavio]

What

Manager.Dial now returns the error when opening a yamux stream fails, instead of logging it and returning a nil connection with a nil error.

Why

The V2 branch logged the failed session.Open() and then fell through to the shared success return:

conn, err := session.Open()
if err != nil {
    log.WithFields(...).WithError(err).Error("failed to open yamux stream for reverse connection")
}

return conn, TransportVersion2, nil

conn is a nil *yamux.Stream boxed in a net.Conn, so the interface is non-nil and the caller's err != nil check passes. The first method call against it panics. A session that is shutting down — the ordinary case when a device drops while a request is in flight — becomes a crashed request rather than a failed one.

Found while investigating #6777; unrelated to that fix and based on master, so the two can merge in either order.

Testing

The new test stores a closed yamux session in the manager and asserts Dial reports the failure. Without the fix it returns (nil, TransportVersion2, nil) and the assertions on the error and the version both fail.

[github-actions]

Claude finished @otavio's task in 1m 38s —— View job

---

Code Review Complete

Reviewed 2 files across code quality, security, testing, language patterns, and architecture — no new issues found. The code looks good as-is.

The fix at server/ssh/pkg/dialer/manager.go:199 correctly mirrors the existing v1 error path (lines 174–181): instead of falling through to return conn, TransportVersion2, nil with a boxed-nil *yamux.Stream, it now returns nil, TransportVersionUnknown, err. This eliminates the panic-on-first-call for the common device-shutdown-mid-request case.

The regression test is deterministic — session.Close() puts the yamux session into shutdown state, so the subsequent session.Open() returns ErrSessionShutdown synchronously with no network round-trip or timing dependency. The stored *yamux.Session type-asserts correctly into the v2 branch, imports are all used, and the assertions match the corrected return values.

---

To request another review round, comment /review.