Add conversation history support to chat API

- Added HISTORY_TURNS env variable
- Updated chat request data creation
- Modified server to handle history
- Added history to test cases

.env.example

@@ -43,6 +43,7 @@ MAX_ASYNC=4
 MAX_TOKENS=32768
 EMBEDDING_DIM=1024
 MAX_EMBED_TOKENS=8192
+#HISTORY_TURNS=3
 
 # Security (empty for no key)
 LIGHTRAG_API_KEY=your-secure-api-key-here