Skip _vector_chunks rename for non-FLAT vec0 columns (#294)

vec0Rename emits an unconditional ALTER TABLE on `<name>_vector_chunks%02d`
for every vector column, but non-FLAT columns (rescore, IVF, DiskANN) don't
create that shadow table — so ALTER TABLE RENAME on a DiskANN-indexed (or
rescore/IVF) vec0 table fails with `no such table` and leaves any cached
prepared statements still referencing the old name.

Mirror the guard already used at create time in vec0_init around
VEC0_SHADOW_VECTOR_N_CREATE: only rename `_vector_chunks` when the column's
index_type is VEC0_INDEX_TYPE_FLAT.

Adds a regression test exercising rename on a DiskANN-indexed table.