Remove synthetic/structured data generation from diskann-providers

[Copilot]

diskann-providers contained grid point generation and synthetic label generation that already have replacements elsewhere: diskann::graph::test::synthetic::Grid and diskann-tools::utils::generate_synthetic_labels_utils respectively.

Deleted

• diskann-providers/src/utils/generate_structured_data.rs — 1D/3D/4D grid vectors, adjacency lists, circle vectors
• diskann-providers/src/utils/generate_synthetic_labels_utils.rs — zipf/random/one_per_point label generation

Grid data migration

All test consumers now use the Grid enum from diskann::graph::test::synthetic:

// Before
let mut adjacency_lists = utils::genererate_3d_grid_adj_list(grid_size as u32);
let mut vectors = f32::generate_grid(dim, grid_size); // called per-type generate_*d_grid_vectors_*

// After
let grid = dim_to_grid(dim);
let mut adjacency_lists = grid.neighbors(grid_size);
let mut vectors = f32::generate_grid(dim, grid_size); // now delegates to Grid::data() internally

GenerateGrid trait impls now wrap Grid::data() / Grid::data_as() instead of the deleted functions. Added diskann = { features = ["testing"] } to dev-dependencies to access the Grid type.

Synthetic labels migration

diskann-tools/src/bin/generate_synthetic_labels.rs now uses diskann_tools::utils::generate_labels with FileStorageProvider instead of the removed diskann_providers version.