chore: add #[must_use] to error type constructors

Add #[must_use] attribute to CodecError::new() and ValidationError::new()
to ensure callers use the returned error value, consistent with other
constructor methods in the codebase.

Author: refcell

crates/rpc/src/validator.rs

@@ -22,6 +22,7 @@ pub struct ValidationError {
 
 impl ValidationError {
     /// Create a new validation error.
+    #[must_use]
     pub fn new(code: i64, message: impl Into<String>) -> Self {
         Self { code, message: message.into() }
     }

crates/traits/src/codec.rs

@@ -11,6 +11,7 @@ pub struct CodecError(pub String);
 
 impl CodecError {
     /// Create a new codec error.
+    #[must_use]
     pub fn new(msg: impl Into<String>) -> Self {
         Self(msg.into())
     }