feat(fts): support custom stop-word lists (#3734)

## What

Expose custom FTS stop-word lists in the Python and TypeScript public
APIs, including their standalone tokenize helpers and remote index
creation.

This PR supports concrete string lists only. It does not add file or
LanceDB-table stop-word sources.

## Why

Rust already exposes Lance's custom stop-word list option. The Python
and TypeScript APIs did not pass it through, and local index details did
not retain the full tokenizer parameters needed by index-backed
tokenization after reopening a table.

## How

- Add `custom_stop_words` / `customStopWords` to the Python and
TypeScript FTS and tokenize options.
- Preserve `None` / `undefined`, empty lists, and list contents without
normalization.
- Load the persisted FTS segment parameters when returning local index
details.
- Serialize the concrete list in remote create-index requests.
- Keep Python and TypeScript tests thin; behavior, persistence, query
tokenization, and remote JSON coverage live primarily in Rust.

## Validation

- `cargo check --quiet --features remote --tests --examples`
- `cargo clippy --quiet --features remote --tests --examples`
- `cargo test --quiet --features remote --tests`
- Python extension rebuild with `uv` and `maturin`
- Targeted Python tests: 4 passed
- Python `ruff format --check` and `ruff check`
- TypeScript build, typecheck, Biome lint, generated docs, and targeted
tests

---------

Co-authored-by: Yang Cen <yangcen@Yangs-Mac-mini.local>

docs/src/python/python.md

@@ -94,9 +94,33 @@ of raw SQL strings with [where][lancedb.query.LanceQueryBuilder.where] and
 
 ## Full text search
 
-Use [lancedb.table.Table.create_fts_index][] for the synchronous API or
-[lancedb.table.AsyncTable.create_index][] with [lancedb.index.FTS][] for the
-asynchronous API.
+Pass `custom_stop_words` to [lancedb.index.FTS][]:
+
+```python
+from lancedb.index import FTS
+
+table.create_index(
+    "text",
+    config=FTS(remove_stop_words=True, custom_stop_words=["acme", "internal"]),
+)
+```
+
+The list replaces the built-in stop words and is used only when
+`remove_stop_words=True`:
+
+- `custom_stop_words=None` uses the built-in list for `language`.
+- `custom_stop_words=[]` removes no words.
+- Values are passed through without trimming, lowercasing, or other rewriting.
+
+The same option is available on `lancedb.tokenize(...)` and the deprecated
+[lancedb.table.Table.create_fts_index][] compatibility helper:
+
+```python
+import lancedb
+
+tokens = list(lancedb.tokenize("acme makes searchable data",
+                               custom_stop_words=["acme"]))
+```
 
 ::: lancedb.index.FTS