krb5: handle GSS-Proxy credentials lifetime

krb5: handle GSS-Proxy credentials lifetime

GSS-Proxy stores its credential in encrypted form in the Kerberos ccache
with a start and end time of 0 and a server principal in the realm named
'X-GSSPROXY:'. This credential is accessed through GSS-Proxy interposer
mechanism in MIT Kerberos and cannot be analysed with raw krb5 API.

As MIT Kerberos has no krb5_cc_get_lifetime() implementation, add check
for the GSS-Proxy credential to smb_krb5_cc_get_lifetime() wrapper to
return KRB5_PLUGIN_NO_HANDLE. The two places where
smb_krb5_cc_get_lifetime() is used then handle this return code to avoid
deciding on the 'expired' lifetime to cause a kinit.

This fixes FreeIPA use case where an IPA API endpoint uses Samba Python
bindings with a GSS-Proxy-controlled credential cache.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=15902

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Pavel Filipenský <pfilipensky@samba.org>
Autobuild-User(master): Pavel Filipensky <pfilipensky@samba.org>
Autobuild-Date(master): Wed Sep  3 10:15:50 UTC 2025 on atb-devel-224