fix(node): reject unsupported FTS languages without panicking (#4369)

`Index.fts({ language: "english" })` and other unsupported language
values abort the Node process because the native factory unwraps the
error returned by `FtsIndexBuilder::language`.

Convert that error into a catchable JavaScript `Error`, using the same
message as `tokenize` and Python. Add regression coverage for
`"Klingon"` and lowercase `"english"`, plus a valid English stemming and
search case across Arrow 15–18.

Validation:
- Native and TypeScript builds: `pnpm build:debug --profile ci` and
`pnpm tsc`.
- Full-text and tokenization tests: `pnpm test __test__/table.test.ts
--runInBand --testNamePattern 'full text|FTS|tokeniz'` — 69 passed.
- Rust formatting and lint: `cargo fmt --all`; `cargo clippy --profile
ci --target x86_64-unknown-linux-gnu -p lancedb-nodejs -- -D warnings`.
- TypeScript formatting and lint: `pnpm exec biome format --write
__test__/table.test.ts` and `pnpm lint`; `pnpm run docs` passed without
generated changes.

Fixes #4367

<!-- lance-gatekeeper-fix:v1 agent=271bb96150580678f4e03b8e36beca03
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

nodejs/src/index.rs

@@ -239,7 +239,12 @@ impl Index {
             opts = opts.base_tokenizer(base_tokenizer);
         }
         if let Some(language) = language {
-            opts = opts.language(&language).unwrap();
+            opts = opts.language(&language).map_err(|_| {
+                napi::Error::from_reason(format!(
+                    "LanceDB does not support the requested language: '{}'",
+                    language
+                ))
+            })?;
         }
         if let Some(max_token_length) = max_token_length {
             opts = opts.max_token_length(Some(max_token_length as usize));