test: add serial_test and cleanup to conversation_service tests

The conversation_service tests were failing in CI due to shared global
DeviceStorage state. When tests run in parallel, they interfere with
each other because they all share the same conversations/index.json.

This fix:
- Adds #[serial] attribute to all conversation_service tests to ensure
  sequential execution
- Adds a cleanup_conversations helper function
- Cleans up existing conversations before tests that depend on counts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: AlexMikhalev

crates/terraphim_service/src/conversation_service.rs

@@ -238,10 +238,23 @@ impl Default for ConversationService {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use serial_test::serial;
     use terraphim_persistence::DeviceStorage;
     use terraphim_types::ChatMessage;
 
+    /// Helper function to clean up all conversations before a test
+    async fn cleanup_conversations(service: &ConversationService) {
+        let all = service
+            .list_conversations(ConversationFilter::default())
+            .await
+            .unwrap_or_default();
+        for conv in all {
+            let _ = service.delete_conversation(&conv.id).await;
+        }
+    }
+
     #[tokio::test]
+    #[serial]
     async fn test_create_and_get_conversation() {
         // Initialize memory-only storage for testing
         let _ = DeviceStorage::init_memory_only().await.unwrap();
@@ -258,12 +271,16 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn test_list_and_filter_conversations() {
         // Initialize memory-only storage for testing
         let _ = DeviceStorage::init_memory_only().await.unwrap();
 
         let service = ConversationService::new();
 
+        // Clean up any existing conversations first
+        cleanup_conversations(&service).await;
+
         // Create conversations with different roles
         service
             .create_conversation("Test 1".to_string(), RoleName::new("Role A"))
@@ -297,12 +314,16 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn test_search_conversations() {
         // Initialize memory-only storage for testing
         let _ = DeviceStorage::init_memory_only().await.unwrap();
 
         let service = ConversationService::new();
 
+        // Clean up any existing conversations first
+        cleanup_conversations(&service).await;
+
         service
             .create_conversation("Machine Learning".to_string(), RoleName::new("Test"))
             .await
@@ -318,6 +339,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn test_export_import_conversation() {
         // Initialize memory-only storage for testing
         let _ = DeviceStorage::init_memory_only().await.unwrap();
@@ -346,12 +368,16 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn test_get_statistics() {
         // Initialize memory-only storage for testing
         let _ = DeviceStorage::init_memory_only().await.unwrap();
 
         let service = ConversationService::new();
 
+        // Clean up any existing conversations first
+        cleanup_conversations(&service).await;
+
         let mut conv1 = service
             .create_conversation("Test 1".to_string(), RoleName::new("Role A"))
             .await