fix(chat): use native async LLM call to prevent session reset on message submit

The previous implementation used asyncio.to_thread(llm.complete) to avoid
blocking the event loop, but Chainlit's contextvars context is not reliably
propagated across thread boundaries, causing the session to reset and clear
the message history on each user submission.

Changes:
- Add llm.acomplete() using litellm.acompletion() (native coroutine)
- Replace asyncio.to_thread() in on_message with await llm.acomplete()
- Store the welcome message in state.messages so it is replayed on reconnect
- Persist state to cl.user_session immediately after appending the user's
  message (before the LLM call) so reconnect detection has the latest history
- Add pytest-asyncio dev dependency and asyncio_mode = "auto" config
- Add 6 async tests for acomplete() in tests/test_llm.py

https://claude.ai/code/session_01SUWzMzFvSfWiHXA2p6rPg9

chat_app.py

@@ -18,7 +18,6 @@ Environment variables (see .env.example):
   DATA_DIR             Directory for completed registration JSON  (default: data/)
 """
 
-import asyncio
 import logging
 import uuid
 from datetime import datetime, timezone
@@ -101,6 +100,8 @@ async def on_chat_start() -> None:
     # Brand new session
     session_id = str(uuid.uuid4())
     state = ConversationState(conversation_id=session_id)
+    # Store the welcome in history so it's replayed if the session reconnects.
+    state.messages.append(ChatMessage(role="assistant", content=_WELCOME_DE))
     cl.user_session.set("state", state.to_dict())
     logger.info("Chat session started: %s", session_id)
     await cl.Message(content=_WELCOME_DE).send()
@@ -114,20 +115,20 @@ async def on_message(message: cl.Message) -> None:
     now = datetime.now(timezone.utc).isoformat()
     state.last_activity = now
 
-    # Append parent's message to history
+    # Append parent's message to history and persist immediately so that any
+    # WebSocket reconnect during the LLM call can replay the full conversation.
     state.messages.append(ChatMessage(role="user", content=message.content))
+    cl.user_session.set("state", state.to_dict())
 
     # --- Build system prompt ---
     system = build_system_prompt(_kb, state)
 
-    # --- Call LLM in a thread so the async event loop (and WebSocket) stay alive ---
-    # litellm.completion is synchronous; running it directly in an async handler
-    # blocks the event loop for the full response duration and causes WebSocket
-    # timeouts that trigger on_chat_start again (clearing the screen).
+    # --- Call LLM natively async to keep the event loop free ---
+    # litellm.acompletion() is a true coroutine — it does not block the event
+    # loop and does not require asyncio.to_thread (which can drop Chainlit's
+    # contextvars, causing the session to reset mid-conversation).
     try:
-        full_content = await asyncio.to_thread(
-            llm.complete, _config.ai_model, system, state.messages
-        )
+        full_content = await llm.acomplete(_config.ai_model, system, state.messages)
     except Exception:
         logger.exception("LLM call failed for session %s", state.conversation_id)
         error_text = fallback_message(state.language)