Refactor session ID handling and improve error management

- Introduced `SessionId` newtype for validated session identifiers, ensuring safety against path traversal attacks.
- Updated session repository methods to accept `SessionId` instead of raw strings, enhancing type safety.
- Removed redundant error handling in repository methods by leveraging the new `Error` type from `zesdex_utils`.
- Simplified atomic JSON write operations by eliminating unnecessary error conversions.
- Enhanced integer casting with a new `CastOr` trait for safer narrowing conversions.
- Removed deprecated error handling code and consolidated error types across the codebase.
- Updated HTTP handlers to utilize the new session ID validation, improving overall robustness.

crates/zesdex-cms/src/application/conversation_service.rs

@@ -58,29 +58,21 @@ impl<R: ConversationRepository> ConversationServiceImpl<R> {
 impl<R: ConversationRepository> ConversationService for ConversationServiceImpl<R> {
     /// Load a conversation from disk for the given session.
     ///
-    /// Flow: resolve session dir → delegate to repo.load() → wrap error with context.
+    /// Flow: resolve session dir → delegate to repo.load().
     fn load_conversation(&self, session_id: &str) -> Result<Conversation, ServiceError> {
         tracing::debug!("loading conversation for session {session_id}");
         let dir = self.session_dir(session_id);
-        self.repo.load(&dir).map_err(|e| {
-            ServiceError::Other(format!(
-                "failed to load conversation for session '{session_id}': {e}"
-            ))
-        })
+        self.repo.load(&dir).map_err(ServiceError::Repository)
     }
 
     /// Persist a conversation to disk.
     ///
-    /// Flow: resolve session dir from conv.session_id → delegate to repo.save() → wrap error.
+    /// Flow: resolve session dir from conv.session_id → delegate to repo.save().
     fn save_conversation(&self, conv: &Conversation) -> Result<(), ServiceError> {
         tracing::debug!("saving conversation for session {}", conv.session_id);
         let dir = self.session_dir(&conv.session_id);
-        self.repo.save(&dir, conv).map_err(|e| {
-            ServiceError::Other(format!(
-                "failed to save conversation for session '{}': {e}",
-                conv.session_id
-            ))
-        })
+        self.repo.save(&dir, conv)?;
+        Ok(())
     }
 
     /// Add a message to a conversation and persist immediately.
@@ -94,11 +86,7 @@ impl<R: ConversationRepository> ConversationService for ConversationServiceImpl<
         tracing::debug!("adding message to session {}", conv.session_id);
         conv.push(msg); // append message to in-memory conversation
         let dir = self.session_dir(&conv.session_id);
-        self.repo.save(&dir, conv).map_err(|e| {
-            ServiceError::Other(format!(
-                "failed to persist conversation after adding message for session '{}': {e}",
-                conv.session_id
-            ))
-        })
+        self.repo.save(&dir, conv)?;
+        Ok(())
     }
 }