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/lancedb/arrow.ts

@@ -72,6 +72,30 @@ export type FieldLike =
       metadata?: Map<string, string>;
     };
 
+/**
+ * Create an Arrow field backed by LanceDB's JSON extension type.
+ *
+ * @param name - The field name.
+ * @param nullable - Whether the field accepts null values.
+ * @example
+ * ```ts
+ * import { connect, makeJsonField } from "@lancedb/lancedb";
+ * import { Schema } from "apache-arrow";
+ *
+ * const schema = new Schema([makeJsonField("metadata")]);
+ * const db = await connect("/path/to/database");
+ * await db.createTable("items", [{ metadata: '{"source":"api"}' }], { schema });
+ * ```
+ */
+export function makeJsonField(name: string, nullable = true): Field {
+  return new Field(
+    name,
+    new Utf8(),
+    nullable,
+    new Map([["ARROW:extension:name", "arrow.json"]]),
+  );
+}
+
 export type DataLike =
   | import("apache-arrow").Data
   | {

nodejs/lancedb/index.ts

@@ -72,6 +72,7 @@ export {
 
 export {
   makeArrowTable,
+  makeJsonField,
   MakeArrowTableOptions,
   Data,
   VectorColumnOptions,