fix: validate BytesValue length in KeyValuePair encoding

Fixes #128

Per draft-ietf-moq-transport-15 section 1.4.2, the maximum length of a
BytesValue is 2^16-1 bytes. Previously, the code would accept BytesValues
exceeding this limit during encoding, which could cause protocol violations
when the peer attempts to decode the message. Now checks if the BytesValue
length exceeds u16::MAX before encoding and returns EncodeError::InvalidValue
if the limit is exceeded, matching the existing validation in the decode path.

Author: diegocatalao

moq-transport/src/coding/kvp.rs

@@ -98,6 +98,10 @@ impl Encode for KeyValuePair {
                 if self.key.is_multiple_of(2) {
                     return Err(EncodeError::InvalidValue);
                 }
+                // size of value must be less then 2^16-1 bytes
+                if v.len() > u16::MAX as usize {
+                    return Err(EncodeError::InvalidValue);
+                }
                 self.key.encode(w)?;
                 v.len().encode(w)?;
                 Self::encode_remaining(w, v.len())?;
@@ -271,4 +275,14 @@ mod tests {
         let decoded = KeyValuePairs::decode(&mut buf).unwrap();
         assert_eq!(decoded, kvps);
     }
+
+    #[test]
+    fn encode_bytes_length_exceeded() {
+        let mut buf = BytesMut::new();
+
+        let huge_bytes = vec![0u8; u16::MAX as usize + 1];
+        let kvp = KeyValuePair::new_bytes(1, huge_bytes);
+        let result = kvp.encode(&mut buf);
+        assert!(result.is_err(), "must rejected because bytes > 65535");
+    }
 }