Implement lease_constraints.expires_at on job.submit

[nficano]

ARCP v1.1 §9.5 introduces optional lease_constraints on job.submit carrying expires_at for time-bounded authority. Swift SDK lacks this.

Steps:

1. Add LeaseConstraints struct with expiresAt: Date?
2. Wire into JobSubmit payload as optional field
3. Runtime enforces: if now > expiresAt, raise LEASE_EXPIRED
4. Samples/LeaseExpiresAt/ mirroring TS
5. Tests

Reference: spec §9.5, §13.4.

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)