test(integration): share indexes across data_plane_extended tests via module fixtures

Replace 40 per-test index creates with 4 module-scoped shared indexes
(dim2/cosine, dim3/cosine, dim4/dotproduct, dim4/cosine). Each test gets
its own UUID namespace for isolation. 3 tests kept as per-test indexes:
sparse lifecycle (unique vector_type=sparse shape) and
test_query_filter_reflects_metadata_update_grpc (unique dim2/dotproduct shape).

Author: jhamon