feat(python): add IVF_HNSW_FLAT vector index support (#3366)

## Summary

Wire up `IVF_HNSW_FLAT` in the Rust core and Python SDK. The index was
documented at https://docs.lancedb.com/indexing/vector-index but
`lancedb.Table.create_index(index_type="IVF_HNSW_FLAT")` raised
`ValueError: Unknown index type IVF_HNSW_FLAT` — the underlying
`pylance` already accepted it, only the LanceDB wrapper was missing the
wiring.

**Rust core (`rust/lancedb`):**
- Add `Index::IvfHnswFlat` / `IndexType::IvfHnswFlat` variants and the
`IvfHnswFlatIndexBuilder` (modelled on `IvfHnswSqIndexBuilder`).
- Build Lance params via the existing `VectorIndexParams::ivf_hnsw(...)`
helper, keeping symmetry with the other `IVF_HNSW_*` variants.
- Forward the variant in `RemoteTable::create_index` and add two
parametrised tests (default + customised config) for the JSON
serialisation.
- New `NativeTable` integration test
(`test_create_index_ivf_hnsw_flat`).

**Python binding (`python/`):**
- New `HnswFlat` dataclass + backwards-compat `IvfHnswFlat` alias.
- PyO3 `extract_index_params` recognises the `HnswFlat` config.
- `LanceTable.create_index(index_type="IVF_HNSW_FLAT", …)` and the sync
`RemoteTable.create_index` both dispatch to the new config.
- `IndexStatistics.index_type` `Literal` and `_lancedb.pyi` stubs cover
the new type so `pyright`/`make check` stays clean.
- Async integration tests (`HnswFlat` + `IvfHnswFlat` alias) and a sync
dispatcher test, mirroring the existing `IVF_HNSW_SQ` coverage.
- Existing `test_index_statistics_index_type_lists_all_supported_values`
updated to include `IVF_HNSW_FLAT`.

A matching Node.js / TypeScript binding is in a follow-up PR.

Closes #3331

## Test plan

- [ ] \`cargo check --quiet --features remote --tests --examples\`
- [ ] \`cargo test --quiet --features remote -p lancedb\` (covers the
new \`test_create_index_ivf_hnsw_flat\` and the two new parametrised
\`RemoteTable::create_index\` cases)
- [ ] \`cargo fmt --all\` / \`cargo clippy --quiet --features remote
--tests --examples\`
- [ ] \`cd python && make develop && make check && make test\` (covers
the two new async tests, the alias test, the dispatcher test, and the
updated \`test_index_statistics_index_type_lists_all_supported_values\`
assertion)

python/src/index.rs

@@ -1,11 +1,13 @@
 // SPDX-License-Identifier: Apache-2.0
 // SPDX-FileCopyrightText: Copyright The LanceDB Authors
 
-use lancedb::index::vector::{IvfFlatIndexBuilder, IvfRqIndexBuilder, IvfSqIndexBuilder};
+use lancedb::index::vector::{
+    IvfFlatIndexBuilder, IvfHnswFlatIndexBuilder, IvfHnswPqIndexBuilder, IvfHnswSqIndexBuilder,
+    IvfPqIndexBuilder, IvfRqIndexBuilder, IvfSqIndexBuilder,
+};
 use lancedb::index::{
     Index as LanceDbIndex,
     scalar::{BTreeIndexBuilder, FtsIndexBuilder},
-    vector::{IvfHnswPqIndexBuilder, IvfHnswSqIndexBuilder, IvfPqIndexBuilder},
 };
 use pyo3::IntoPyObject;
 use pyo3::types::PyStringMethods;
@@ -162,8 +164,26 @@ pub fn extract_index_params(source: &Option<Bound<'_, PyAny>>) -> PyResult<Lance
                 }
                 Ok(LanceDbIndex::IvfHnswSq(hnsw_sq_builder))
             }
+            "HnswFlat" => {
+                let params = source.extract::<IvfHnswFlatParams>()?;
+                let distance_type = parse_distance_type(params.distance_type)?;
+                let mut hnsw_flat_builder = IvfHnswFlatIndexBuilder::default()
+                    .distance_type(distance_type)
+                    .max_iterations(params.max_iterations)
+                    .sample_rate(params.sample_rate)
+                    .num_edges(params.m)
+                    .ef_construction(params.ef_construction);
+                if let Some(num_partitions) = params.num_partitions {
+                    hnsw_flat_builder = hnsw_flat_builder.num_partitions(num_partitions);
+                }
+                if let Some(target_partition_size) = params.target_partition_size {
+                    hnsw_flat_builder =
+                        hnsw_flat_builder.target_partition_size(target_partition_size);
+                }
+                Ok(LanceDbIndex::IvfHnswFlat(hnsw_flat_builder))
+            }
             not_supported => Err(PyValueError::new_err(format!(
-                "Invalid index type '{}'.  Must be one of BTree, Bitmap, LabelList, FTS, IvfPq, IvfSq, IvfHnswPq, or IvfHnswSq",
+                "Invalid index type '{}'.  Must be one of BTree, Bitmap, LabelList, FTS, IvfPq, IvfSq, IvfHnswPq, IvfHnswSq, or IvfHnswFlat",
                 not_supported
             ))),
         }
@@ -250,6 +270,17 @@ struct IvfHnswSqParams {
     target_partition_size: Option<u32>,
 }
 
+#[derive(FromPyObject)]
+struct IvfHnswFlatParams {
+    distance_type: String,
+    num_partitions: Option<u32>,
+    max_iterations: u32,
+    sample_rate: u32,
+    m: u32,
+    ef_construction: u32,
+    target_partition_size: Option<u32>,
+}
+
 #[pyclass(get_all)]
 /// A description of an index currently configured on a column
 pub struct IndexConfig {