CDP-5981: add v2 batch endpoint to TrackClient

[jcpsimmons]

Summary

Adds TrackClient.batch(operations) which POSTs an array of operations to the v2 batch endpoint. The v2 URL is derived from the existing track root (or a custom defaults.url) by swapping /api/v1 → /api/v2, so region selection and custom URL overrides continue to work without additional configuration.

API

cio.batch([
  { type: "person", action: "identify", identifiers: { id: "1" }, attributes: { plan: "pro" } },
  { type: "person", action: "event", identifiers: { id: "1" }, name: "signup" },
]);

The operations parameter is required and must be a non-empty array; otherwise a MissingParamError is thrown. Operation items are passed through as the batch field of the request body, so callers are free to shape items per the public API docs without the SDK pinning a schema snapshot.

Test plan

• npm run build — clean
• npm test — 156 tests pass, nyc 100% coverage gate holds (branches/lines/functions/statements)
• Constructor tests extended to assert the new trackV2Root field for default region, both regions, and custom URL
• #batch tests cover the happy path (US), region variant (EU), empty-array rejection, and non-array rejection

Opening as draft pending review of the URL derivation strategy and the permissive BatchOperation type.

---

Note

Low Risk
New optional API surface with input validation only; no changes to existing identify/track flows beyond URL metadata on the client.

Overview
Adds TrackClient.batch(operations) to POST a non-empty array of operations to the Track API /api/v2/batch endpoint, wrapping them as { batch: operations } with permissive BatchOperation typing.

Introduces trackV2Root (and Region.trackV2Url) by deriving v2 from the existing v1 track URL—/api/v1 → /api/v2 for built-in US/EU regions and custom defaults.url when that segment is present; a custom URL without /api/v1 is used unchanged for both roots.

README documents usage; tests cover US/EU batch URLs, v2 root construction, and validation when operations is missing, empty, or not an array.

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