feat(proto): enforce maximum length for server fill parameter

Move MAX_SERVER_FILL_BYTES to irtt-proto and validate it during decoding to prevent processing oversized parameters.

crates/irtt-client/src/config.rs

@@ -1,12 +1,12 @@
 use std::{net::SocketAddr, time::Duration};
 
+pub use irtt_proto::MAX_SERVER_FILL_BYTES;
 use irtt_proto::{Clock, ReceivedStats, StampAt};
 
 pub(crate) const DEFAULT_PORT: u16 = 2112;
 pub const MAX_DSCP_CODEPOINT: u8 = 63;
 pub const MAX_TTL: u32 = 255;
 pub const MAX_UDP_PAYLOAD_LENGTH: u32 = 65_507;
-pub const MAX_SERVER_FILL_BYTES: usize = 32;
 pub(crate) const DEFAULT_DURATION: Duration = Duration::from_secs(3);
 pub(crate) const DEFAULT_INTERVAL: Duration = Duration::from_secs(1);
 pub(crate) const DEFAULT_OPEN_TIMEOUTS: [Duration; 4] = [