ssh: enforce nil Permissions when returning PartialSuccessError

In serverAuthenticate, the permissions variable is reset to nil at the
beginning of the authentication loop. If an authentication callback
returns a PartialSuccessError along with non-nil Permissions, those
permissions are currently silently discarded before the next
authentication step.

This change returns an error if a callback returns both a PartialSuccessError
and non-nil Permissions, preventing API misuse where the user might
erroneously expect those permissions to be preserved or merged into the
final session permissions.

This issue was found during a security audit by NCC Group Cryptography
Services, sponsored by Teleport.

Fixes golang/go#79562
Fixes CVE-2026-39828

Change-Id: I632c9e46e2b5e8804ef88081063a3612a2462f9f
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781621
Reviewed-by: Neal Patel <nealpatel@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>