feat: Add `apns-id` header to client return value

[RedPhoenixQ]

This exposes the apns-id header that is returned to identify a notification for debugging and tracking.

Summary by CodeRabbit

• New Features

  • APNs notification IDs returned by the server are now captured, surfaced with results, and included in subsequent request headers when available.

• Tests

  • Test suite expanded to verify APNs ID extraction/merging into results and correct request/connection behavior under concurrent requests.

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