'all_varchar = true' output data with read_xlsx inconsistent with 'FIELD_TYPES=STRING' using st_read for excel files

DuckDBs excel extension read_xlsx function with 'all_varchar = true' renders data that is less malleable in term of type casting the string data after the initial read than what is produced from using the st_read with 'FIELD_TYPES=STRING'

I made an issue on [ibis project](https://github.com/ibis-project/ibis) linked here [#11046](https://github.com/ibis-project/ibis/issues/11046) that goes into more detail. On that ibis issue it was suggested that this may be something that can be looked at upstream in DuckDBs excel extension.

Start by reviewing the DuckDB Excel extension's read_xlsx behavior with all_varchar = true and compare it with st_read using FIELD_TYPES=STRING. Use ibis issue #11046 for reproduction details and clarify which string representations and casts differ. Done means the two reading approaches have consistent, usable casting behavior with regression coverage.

Written by the indexing model from the issue text.