fix: MDEV-40651 ALTER TABLE and DDL in general is now executed in an atomic manner b/c previously MariaDB DDL triggered multiple autocommit DDL statements in DuckDB some of those can fail leaving table in unsable state.

[drrtuy]

Make DuckDB ALTER TABLE operations atomic

What

Fix MDEV-40651 by executing all DuckDB statements generated for a single MariaDB ALTER TABLE within one transaction. This prevents partial schema changes when a later DDL statement fails.

Key changes

• Wrap generated DuckDB ALTER statements in BEGIN/COMMIT and roll back on failure.
• Preserve MariaDB implicit values when adding NOT NULL columns, then remove temporary defaults where required.
• Reject partitioned DuckDB tables via HTON_NO_PARTITION.
• Add MTR coverage for atomic rollback, implicit defaults, and partition rejection.
• Update DuckDB architecture and incompatibility documentation.

How to test

Run the DuckDB MTR test:

./mtr duckdb.mdev_40651