Apply lease_constraints.expires_at even when no lease_request capabilities are supplied

[nficano]

Category: bug Severity: minor
Location: lib/arcp/runtime/job_manager.rb:256-269
Spec: ARCP v1.1 §7.1, §9.5

What

build_lease returns nil unless submit.lease_request is present. A client that supplies lease_constraints (e.g. expires_at) but no lease_request gets no lease at all, so the requested expiration is silently dropped and LeaseManager never registers an expiry to enforce. Spec §7.1 documents lease_constraints as an independent submit field, and §9.5 ties enforcement to it; the SDK only honors it as a side effect of also requesting capabilities. The expiry the caller asked for vanishes with no error.

Evidence

# lib/arcp/runtime/job_manager.rb:256-269
def build_lease(submit, job_id)
  return nil unless submit.lease_request    # constraints-only submit yields no lease, no expiry

  submit.lease_constraints&.enforce_max_budget!(submit.lease_request.budget)
  Arcp::Lease::Lease.new(
    id: "lse_#{job_id}",
    capabilities: submit.lease_request.capabilities,
    # ...
    expires_at: submit.lease_constraints&.expires_at || submit.lease_request.expires_at,
    issued_at: @clock.now.iso8601
  )
end

Proposed fix

Build a lease whenever lease_request or lease_constraints is present (a constraints-only lease carries empty capabilities plus the expires_at), so the expiration is registered and enforceable. Add a test submitting lease_constraints with a future expires_at and no lease_request, asserting the accepted lease carries expires_at and that LeaseManager would expire it.

Acceptance criteria

• A constraints-only submit produces a lease whose expires_at matches the request.
• The lease manager enforces expiry for such jobs.
• job.accepted echoes the expires_at.