libssh backend CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256 validation bypass

## Summary: If libcurl is built against libssh `CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256` is quietly ignored. As a result a SSH connection will be established even if the SHA256 key set doesn't match. ## Steps To Reproduce: 1. configure libcurl with libssh and build it 2. `curl --hostpubsha256 HOSTFINGERPRINTHERE sftp://example.tld/` Instead of failing due to mismatching fingerprint the connection quietly continues. While the `CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256 ` documentation does mention that this option `Requires the libssh2 backend`, it is still wrong to quietly ignore the validation. ## Remediation Change `lib/vssh/libssh.c` `myssh_is_known` to reject connection if `CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256` is set, or implement sha256 fingerprint support for libssh. ## Impact SSH host validation bypass.