fix: handle string input in _get_further_suggestion for backward comp…

…atibility

Fixes #1215

## Root Cause Analysis
- API endpoint /suggestions may receive string-type message
- _get_further_suggestion expected MessageList (list[dict])
- When string passed, string[-2:] returns last 2 chars
- Then msg['role'] causes TypeError: string indices must be integers

## Solution
**Short-term (this PR):** Add type check and auto-convert string to MessageList format

**Long-term (follow-up):** Add API input validation layer

## Changes
- Updated type hint: MessageList | str
- Added isinstance check for backward compatibility
- Log warning when conversion happens

## Test Plan
- [x] String input is converted to [{"role": "user", "content": ...}]
- [x] MessageList input still works
- [x] Warning logged for string input
- [x] No more TypeError

## Impact
- Fixes runtime crash
- Backward compatible
- Graceful degradation

src/memos/api/handlers/suggestion_handler.py

@@ -25,19 +25,24 @@
 
 def _get_further_suggestion(
     llm: Any,
-    message: MessageList,
+    message: MessageList | str,
 ) -> list[str]:
     """
     Get further suggestion based on recent dialogue.
 
     Args:
         llm: LLM instance for generating suggestions
-        message: Recent chat messages
+        message: Recent chat messages (MessageList or string for backward compatibility)
 
     Returns:
         List of suggestion queries
     """
     try:
+        # Handle backward compatibility: convert string to MessageList format
+        if isinstance(message, str):
+            logger.warning("Received string message, converting to MessageList format")
+            message = [{"role": "user", "content": message}]
+
         dialogue_info = "\n".join([f"{msg['role']}: {msg['content']}" for msg in message[-2:]])
         further_suggestion_prompt = FURTHER_SUGGESTION_PROMPT.format(dialogue=dialogue_info)
         message_list = [{"role": "system", "content": further_suggestion_prompt}]