Fix JSON serialization to handle non-ASCII characters in _convert_to_content

Author: detteiu0330

src/mcp/server/fastmcp/server.py

@@ -551,7 +551,9 @@ 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)