fix: preserve non-ASCII characters in JSON output

Add ensure_ascii=False parameter to json.dumps() when converting results 
to JSON strings in _convert_to_content function. This prevents Unicode 
characters (like Chinese) from being escaped as \uXXXX sequences, making 
the output more readable and user-friendly for multilingual content.

Author: VvvvvGH

src/mcp/server/fastmcp/server.py

@@ -552,7 +552,7 @@ def _convert_to_content(
 
     if not isinstance(result, str):
         try:
-            result = json.dumps(pydantic_core.to_jsonable_python(result))
+            result = json.dumps(pydantic_core.to_jsonable_python(result), ensure_ascii=False)
         except Exception:
             result = str(result)