feat: enhance HTTP/2 support by ensuring Host header is set and adding multiplexed request tests

rust/crates/rustproxy-passthrough/src/sni_parser.rs

@@ -196,6 +196,7 @@ pub fn is_http(data: &[u8]) -> bool {
         b"PATC",
         b"OPTI",
         b"CONN",
+        b"PRI ",   // HTTP/2 connection preface
     ];
     starts.iter().any(|s| data.starts_with(s))
 }