Function validate_proxy_auth 

pub fn validate_proxy_auth(
    header_bytes: &[u8],
    session_token: &Zeroizing<String>,
) -> Result<()>

Validate a Proxy-Authorization header against the session token.

Accepts two formats:

• Proxy-Authorization: Bearer <token> (nono-aware clients)
• Proxy-Authorization: Basic base64(nono:<token>) (standard HTTP clients like curl)

Case-insensitive header name and scheme matching per HTTP spec.