## 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.
