Validate create_note/create_task title is a string before DB insert

The model occasionally passes title as a list (e.g. when asked to create
multiple notes at once), causing asyncpg DataError on the INSERT. Return a
clear error result so the model sees the problem and retries with individual
calls instead of crashing with an unhandled exception.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/services/tools.py

@@ -740,6 +740,10 @@ async def execute_tool(user_id: int, tool_name: str, arguments: dict) -> dict:
         if tool_name == "create_task":
             task_title = arguments.get("title", "Untitled Task")
             task_body = arguments.get("body", "")
+            if not isinstance(task_title, str):
+                return {"success": False, "error": "title must be a string. Call create_task once per task."}
+            if not isinstance(task_body, str):
+                task_body = ""
             note = await create_note(
                 user_id=user_id,
                 title=task_title,
@@ -767,6 +771,12 @@ async def execute_tool(user_id: int, tool_name: str, arguments: dict) -> dict:
             note_title = arguments.get("title", "Untitled Note")
             note_body = arguments.get("body", "")
             note_tags = arguments.get("tags", [])
+            if not isinstance(note_title, str):
+                return {"success": False, "error": "title must be a string. Call create_note once per note."}
+            if not isinstance(note_body, str):
+                note_body = ""
+            if not isinstance(note_tags, list):
+                note_tags = []
             note = await create_note(
                 user_id=user_id,
                 title=note_title,