Add subnet_uuid to droplets API responses

[Copilot]

Droplet responses expose vpc_uuid but not the VPC subnet the Droplet is attached to. This adds a subnet_uuid field alongside it.

Changes

• specification/resources/droplets/models/droplet.yml: new optional subnet_uuid string property next to vpc_uuid, documented with the vpc:read scope requirement. This model backs all droplet responses (list, get, create single/multi, neighbors), so the field propagates everywhere.
• specification/resources/droplets/responses/examples.yml: subnet_uuid added to every droplet example, using a stable subnet UUID per distinct vpc_uuid so examples stay internally consistent.

vpc_uuid:
  type: string
  example: '760e09ef-dc84-11e8-981e-3cfdfeaae000'
  description: A string specifying the UUID of the VPC to which the Droplet
    is assigned.<br>Requires `vpc:read` scope.

subnet_uuid:
  type: string
  example: '9c7a0d63-3c4e-4d3d-9f56-3b0fb0e5e4a1'
  description: A string specifying the UUID of the VPC subnet to which the
    Droplet is assigned.<br>Requires `vpc:read` scope.

Request models (droplet_create.yml) are untouched — this is response-only, per the problem statement.