command: model VehicleData field 18 as an opaque bytes payload

[Bre77]

Intent

• Live vehicleDataSubscription pushes carry a VehicleData field 18 (~137 bytes) that this package's VehicleData message doesn't define at all - a real proto-version gap, confirmed from our own observations and contributions from the community.
  • The field's wire shape (length-delimited, i.e. a nested message) is confirmed, but a field-by-field decode of its contents wasn't preserved, so it isn't published as a named message type here - that would mean guessing at names/semantics we can't back up.
  • Modelled as an opaque bytes unknown = 18;, matching the existing unknown naming convention already used elsewhere in this file for undocumented payloads. Wire-compatible: any consumer that already skips unknown fields is unaffected, and this just gives it a name and round-trips the raw bytes for anyone who wants to inspect them.
  • Naming/typing it more precisely is left for a follow-up once a full capture is available.
• Regenerated both language packages; buf lint, buf breaking, and the upstream coverage gate are all green.
• Adds a minor changeset (new field = wire-compatible addition).