ssh: enforce the source-address critical option for all auth callbacks

CVE-2026-46595 extended source-address validation, historically applied
only to the Permissions returned by PublicKeyCallback, to the ones
returned by VerifiedPublicKeyCallback. The documented contract of
Permissions.CriticalOptions does not restrict enforcement to a specific
authentication method, so move the check to a single point at the end of
each authentication attempt, where it covers the Permissions returned by
any callback (password, keyboard-interactive, none and GSSAPI included).

The check at public key cache insertion time is kept: it remains
authoritative for the Permissions returned by PublicKeyCallback, which
VerifiedPublicKeyCallback may replace before the check at the end of the
authentication attempt runs and which are not re-checked on partial
success. It also still makes public key queries fail before the client
produces a signature when PublicKeyCallback supplies the restriction.

Fixes CVE-2026-56854
Fixes golang/go#80213

Change-Id: Ibd612a8e4240bd710e33754f3ceb95bb29169d9a
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/797040
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>