fix(infra): require an unused hostname before the billable create

The requested hostname is the durable recovery correlation value when a
create response is unparseable, but the default (nehemiah-metal-01) is
reusable: a second live server with the same hostname makes hostname
recovery ambiguous and can strand a billed host. Query the provider before
creating and refuse to proceed while any live server already carries the
hostname. The test harness covers the collision refusal and the new
provider read.

Co-authored-by: Cursor <cursoragent@cursor.com>