fix(credssp): grammar in expect message + guard empty output token in PubKeyInfo path

[Copilot]

Addresses two review comments from PR #689.

Changes

• Grammar fix: "an decrypt" → "a decrypt" in the expect message inside exchange_pub_key_auth

• Logic fix: The else branch in the Ok/CompleteNeeded acceptor path previously advanced unconditionally to the two-leg PubKeyInfo state whenever pub_key_auth was absent — even when the SSPI output token was empty. An empty token means there is nothing to forward to the client; sending empty negoTokens and advancing state is incorrect. The fix gates the two-leg path on a non-empty token; an empty token falls back to the original single-leg error (requiring pubKeyAuth in the same TSRequest).

} else {
    let output = output_token.remove(0).buffer;
    if !output.is_empty() {
        ts_request.nego_tokens = Some(output);
        self.state = CredSspState::PubKeyInfo;
    } else {
        try_cred_ssp_server!(
            Err(Error::new(
                ErrorKind::InvalidToken,
                String::from("CredSSP server expected an encrypted public key"),
            )),
            ts_request
        );
    }
}

[Benoît Cortier (CBenoit)]

Pavlo Myroniuk (@PavloMyroniuk-apriorit) Automatically fixed the Copilot feedback in the last PR merged