Category aggregation functions

For aggregation functions, there should be window functions (window only), as subset of aggregation functions, is not allowed to used without a window frame. E.g `select at(col, 0) from t2` is invalid.

_Originally posted by @aceforeverd in https://github.com/4paradigm/OpenMLDB/issues/3010#issuecomment-1413453073_

The issue references a comment from issue #3010 about window aggregation functions. Look for SQL parsing and validation code, likely where aggregation functions are defined and where window frame requirements are enforced. Test the invalid query `select at(col, 0) from t2` to see the current error, then modify validation to reject window-only aggregates without a frame. Check existing tests for aggregation or window functions to understand the expected behavior.

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