fix: add clippy allow for manual_is_multiple_of lint (stable Rust compat)

src/security/secrets.rs

@@ -240,6 +240,7 @@ fn hex_encode(data: &[u8]) -> String {
 }
 
 /// Hex-decode a hex string to bytes.
+#[allow(clippy::manual_is_multiple_of)]
 fn hex_decode(hex: &str) -> Result<Vec<u8>> {
     if hex.len() % 2 != 0 {
         anyhow::bail!("Hex string has odd length");