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/python/tests/test_table.py

@@ -11,7 +11,7 @@ from unittest.mock import patch
 
 import lancedb
 from lancedb.dependencies import _PANDAS_AVAILABLE
-from lancedb.index import HnswPq, HnswSq, IvfPq
+from lancedb.index import HnswFlat, HnswPq, HnswSq, IvfPq
 import numpy as np
 import polars as pl
 import pyarrow as pa
@@ -917,6 +917,21 @@ def test_create_index_method(mock_create_index, mem_db: DBConnection):
         "my_vector", replace=True, config=expected_config, name=None, train=True
     )
 
+    table.create_index(
+        vector_column_name="my_vector",
+        metric="cosine",
+        index_type="IVF_HNSW_FLAT",
+        sample_rate=0.1,
+        m=29,
+        ef_construction=10,
+    )
+    expected_config = HnswFlat(
+        distance_type="cosine", sample_rate=0.1, m=29, ef_construction=10
+    )
+    mock_create_index.assert_called_with(
+        "my_vector", replace=True, config=expected_config, name=None, train=True
+    )
+
 
 @patch("lancedb.table.AsyncTable.create_index")
 def test_create_index_name_and_train_parameters(