feat(python): expose the NGram scalar index

## What kind of change does this PR introduce?

Feature. Exposes the NGram scalar index in the Python SDK.

## What is the current behavior?

The NGram scalar index already exists in the Rust core (`Index::NGram`) and is accepted over the remote protocol (the `"NGRAM"` type), but the Python SDK has no way to create it. `lancedb.index` offers `BTree`, `Bitmap`, `LabelList`, `Fm`, and the vector indexes, but not `NGram`, so it cannot be built through `create_index(config=...)`.

## What is the new behavior?

Adds an `NGram` config to `lancedb.index` and wires it through the pyo3 binding:

```python
from lancedb.index import NGram

await table.create_index("text", config=NGram())
```

The NGram index accelerates substring filters such as `contains(col, 'needle')`, along with some `LIKE` and regular-expression predicates, on UTF-8 string columns. It is parameter free and uses Lance's default trigram parameters.

Changes:

- `lancedb.index`: new `NGram` dataclass, added to `__all__`.
- pyo3 binding (`python/src/index.rs`): a match arm mapping `NGram` to `LanceDbIndex::NGram`, plus the import and the error message.
- Type hints: `NGram` added to the `create_index` config unions and the isinstance check in `table.py`, the `IndexConfigType` union, and the `_lancedb.pyi` stubs; `"NGRAM"` added to the `IndexType` and `IndexStatistics.index_type` literals.
- Test: `test_create_ngram_index` covers creation, `list_indices`, `index_stats`, and that a substring filter uses the index.
- Docs: an NGram section in the scalar index guide.

This mirrors the approach used for the RTree index in https://github.com/lancedb/lancedb/pull/4555 (that PR keeps the RTree scope; this one adds the remaining parameter free string index).

python/src/index.rs

@@ -8,7 +8,9 @@ use lancedb::index::vector::{
 };
 use lancedb::index::{
     Index as LanceDbIndex,
-    scalar::{BTreeIndexBuilder, DocumentGranularity, FmIndexBuilder, FtsIndexBuilder},
+    scalar::{
+        BTreeIndexBuilder, DocumentGranularity, FmIndexBuilder, FtsIndexBuilder, NGramIndexBuilder,
+    },
 };
 use pyo3::IntoPyObject;
 use pyo3::types::PyStringMethods;
@@ -40,6 +42,7 @@ pub fn extract_index_params(source: &Option<Bound<'_, PyAny>>) -> PyResult<Lance
             "Bitmap" => Ok(LanceDbIndex::Bitmap(Default::default())),
             "LabelList" => Ok(LanceDbIndex::LabelList(Default::default())),
             "Fm" => Ok(LanceDbIndex::Fm(FmIndexBuilder::default())),
+            "NGram" => Ok(LanceDbIndex::NGram(NGramIndexBuilder::default())),
             "FTS" => {
                 let params = source.extract::<FtsParams>()?;
                 let mut inner_opts = FtsIndexBuilder::default()
@@ -205,7 +208,7 @@ pub fn extract_index_params(source: &Option<Bound<'_, PyAny>>) -> PyResult<Lance
                 Ok(LanceDbIndex::IvfHnswFlat(hnsw_flat_builder))
             }
             not_supported => Err(PyValueError::new_err(format!(
-                "Invalid index type '{}'.  Must be one of BTree, Bitmap, LabelList, Fm, FTS, IvfPq, IvfSq, IvfHnswPq, IvfHnswSq, or IvfHnswFlat",
+                "Invalid index type '{}'.  Must be one of BTree, Bitmap, LabelList, Fm, NGram, FTS, IvfPq, IvfSq, IvfHnswPq, IvfHnswSq, or IvfHnswFlat",
                 not_supported
             ))),
         }