fix: MDEV-40846 DuckDB handles functionality that is based on invisible columns, e.g. WITH SYSTEM VERSIONING.

[drrtuy]

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Fix MDEV-40846: handle invisible columns in DuckDB pushdown

What

Cross-engine queries touching invisible columns (e.g. tables WITH SYSTEM VERSIONING) produced incorrect results or column-count mismatches when pushed down to DuckDB. This PR makes the pushdown planner recognize invisible columns and fall back to MariaDB execution when it can't safely handle them.

Key changes

• ha_duckdb_pushdown.cc: add Invisible_field_enumerator and query_references_invisible_fields() to bail out of pushdown when a query explicitly references invisible fields (excluding implicit versioning fields).
• Add has_visible_field_prefix() so external tables only push down when invisible columns are strictly trailing.
• Replace silent column-count truncation in next_row() with a hard error when DuckDB's result width doesn't match MariaDB metadata.
• cross_engine_scan.cc: stop binding trailing invisible fields to the DuckDB scan.
• Add cross_engine_join test cases (9–11) covering system versioning plus trailing/non-trailing invisible columns.

How to test

Run mysql-test suite duckdb.cross_engine_join.