No test reaches the grouped batch fold with deletes: a fold that ignored the delete mask passes every suite tried #1675

Found while porting parallel_vector_agg (#1673).

What was measured

A mutant in which the grouped batch fold ignores the delete mask (patch: mut-gnodelete.patch in the port's logs) fails nothing:

• main's parallel_vector_agg.sh: 0 failed;
• the ported shell in test: parallel_vector_agg gains its pytest twin, and two failed arms no longer agree (#1341) #1673: 0 failed.

The twin was not run under it.

Why

Every grouped query in that suite has a text key or an expression key. Those take the row path, so the batch fold never runs there. Deleted rows are therefore never presented to it.

What is unknown

Whether another suite (for example join_vector_agg, the native agg suites, or native_agg_deletes) reaches the grouped batch fold with deleted rows. That has to be checked by running the same mutant across them before a test is written.

Next step

1. Run the mutant over every suite that groups on a columnar table, and record which, if any, goes red.
2. If none does: add an arm that groups on a plain integer key, deletes rows, and compares with core's grouped aggregate (enable_group_vectorization=off). Show it is red under the mutant and green on main, in both twins.