feat(nodejs): add JSON field helper (#4082)

## Issue

Fixes #4063

## Background

The Node.js SDK currently requires callers to know the Arrow extension
metadata needed to represent JSON fields. This makes a common LanceDB
schema type unnecessarily verbose and easy to get wrong.

## Changes

- Add `makeJsonField(name, nullable = true)` to create a UTF-8 Arrow
field with the `arrow.json` extension metadata.
- Re-export the helper from the public Node.js entry point.
- Add coverage for the default nullable behavior, explicit non-nullable
fields, and the extension metadata.
- Add the generated TypeDoc function page and public globals entry,
including a usage example.

## Implementation

The helper uses the existing Apache Arrow `Field` type and sets
`ARROW:extension:name` to `arrow.json`, matching the metadata convention
already used by LanceDB.

## Compatibility

This is an additive Node.js API. Existing schema construction and Arrow
behavior are unchanged.

## Verification

- `pnpm test -- arrow.test.ts --runInBand` — 236 tests passed.
- `pnpm exec biome ci lancedb/arrow.ts lancedb/index.ts
__test__/arrow.test.ts` — passed.
- `git diff --check` — passed.

## Not run / known limitations

- `pnpm build` and `pnpm run docs` were attempted after expanding the
checkout. Both are blocked locally by the native binding build/type
declarations: Cargo did not complete, and TypeDoc reported the missing
generated `nodejs/lancedb/native` module. The docs files were generated
from the updated TypeScript comments; full build and docs validation are
left to CI.

nodejs/__test__/arrow.test.ts

@@ -20,6 +20,7 @@ import {
   fromTableToBuffer,
   makeArrowTable,
   makeEmptyTable,
+  makeJsonField,
 } from "../lancedb/arrow";
 import {
   EmbeddingFunction,
@@ -28,6 +29,21 @@ import {
 import { EmbeddingFunctionConfig } from "../lancedb/embedding/registry";
 import { sanitizeTable } from "../lancedb/sanitize";
 
+it("creates a nullable JSON field with the Arrow extension metadata", () => {
+  const field = makeJsonField("metadata");
+
+  expect(field.name).toBe("metadata");
+  expect(field.type).toEqual(new arrow15.Utf8());
+  expect(field.nullable).toBe(true);
+  expect(field.metadata).toEqual(
+    new Map([["ARROW:extension:name", "arrow.json"]]),
+  );
+});
+
+it("allows JSON fields to be non-nullable", () => {
+  expect(makeJsonField("metadata", false).nullable).toBe(false);
+});
+
 // biome-ignore lint/suspicious/noExplicitAny: skip
 function sampleRecords(): Array<Record<string, any>> {
   return [