refactor(blockchain): derive UnknownTopic via thiserror

Replace the hand-written Display and Error impls with a thiserror derive,
matching the crate convention (store.rs, key_manager.rs, etc.). No behavior
change: the message is identical.

Author: MegaRedHand

crates/blockchain/src/events.rs

@@ -14,7 +14,7 @@ use ethlambda_types::ShortRoot;
 use ethlambda_types::checkpoint::Checkpoint;
 use ethlambda_types::primitives::H256;
 use serde::Serialize;
-use std::{fmt, str::FromStr};
+use std::str::FromStr;
 use tokio::sync::broadcast;
 use tracing::warn;
 
@@ -46,17 +46,10 @@ impl Topic {
 }
 
 /// Error returned by [`Topic::from_str`] for a name matching no topic.
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
+#[error("unknown topic: '{0}'")]
 pub struct UnknownTopic(String);
 
-impl fmt::Display for UnknownTopic {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "unknown topic: '{}'", self.0)
-    }
-}
-
-impl std::error::Error for UnknownTopic {}
-
 impl FromStr for Topic {
     type Err = UnknownTopic;