Offline query output should have schema, not one string column

https://github.com/4paradigm/OpenMLDB/blob/b78fb441aaede27d1a78818d29c475ce1096a197/src/sdk/sql_cluster_router.cc#L2413

The issue points to a specific line in sql_cluster_router.cc where offline query output is handled. Start by examining the code around line 2413 to understand how the output schema is currently generated. Look for related functions that define or return schemas for queries. Check test files for offline query output to see expected behavior. The goal is to modify the output to include proper schema information instead of a single string column.

Written by the indexing model from the issue text.