Skip the key exchange test when using the system's ssh CLI if the required KEX algorithm (e.g., mlkem768x25519-sha256) is not supported. This is determined by running ssh -Q kex and checking for the presence of the target algorithm. Prevents false test failures in CI environments with older or limited SSH implementations. Cq-Include-Trybots: luci.golang.try:x_crypto-gotip-darwin-amd64-longtest,x_crypto-gotip-linux-amd64-longtest,x_crypto-gotip-windows-amd64-longtest Change-Id: I3fac703ec70559e18b30d5fff88274335a7c3952 Reviewed-on: https://go-review.googlesource.com/c/crypto/+/679195 LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Carlos Amedee <carlos@golang.org>