fix: expose InvalidJSONFieldError and NoFormData in multipartErrors

[LeSingh1]

What

Two error constructors defined inside the plugin were missing from the fastify.multipartErrors decorator:

• InvalidJSONFieldError (code FST_INVALID_JSON_FIELD_ERROR) — thrown when a multipart field has Content-Type: application/json but carries invalid JSON
• NoFormData (code FST_NO_FORM_DATA) — thrown when globalThis.FormData is unavailable

The TypeScript MultipartErrors interface was also missing FileBufferNotFoundError, InvalidJSONFieldError, and NoFormData, leaving the type out of sync with the runtime.

Why it matters

Users who want to branch on the error type idiomatically do:

} catch (err) {
  if (err instanceof fastify.multipartErrors.InvalidJSONFieldError) { ... }
}

With InvalidJSONFieldError absent from multipartErrors, that check throws a TypeError: Right-hand side of 'instanceof' is not callable instead of returning false, which is a silent breakage. The error is thrown by the plugin with the documented code, but there was no way to reach its constructor through the public API.

What changed

• index.js: add InvalidJSONFieldError and NoFormData to the fastify.decorate('multipartErrors', …) call
• types/index.d.ts: add FileBufferNotFoundError, InvalidJSONFieldError, and NoFormData to MultipartErrors
• test/multipart-json.test.js:
  • Fix t.plan(2) → t.plan(3) in two existing tests that were silently miscounted (the instanceof check was throwing a TypeError, skipping one assertion, and the wrong plan count went unnoticed)
  • Add a unit test confirming multipartErrors.InvalidJSONFieldError is a function and that an error thrown by the plugin is an instance of it

All tests pass (npm test) including TypeScript type checks.