test(escrow_contract): write integration tests for full escrow lifecycle

[Ogstevyn]

Summary

• Implemented the escrow lifecycle functions required for the tests to compile: create_escrow, release_escrow, refund_escrow, raise_dispute, resolve_dispute, and get_escrow_record
• Added EscrowStatus and EscrowRecord contract types and a typed DataKey::Escrow(u64) storage key
• Every persistent write calls extend_ttl via the constants defined in Smart Contract: Implement Escrow TTL and Storage Ledger Extension #15
• Wrote 12 integration tests covering: happy-path create→release (driver balance asserted), refund path (sender balance restored), dispute resolved to driver, dispute resolved to sender, all four unauthorized-caller failure modes, duplicate delivery ID rejection, invalid state transitions (double-release, release-then-refund), and event emission across the full dispute lifecycle
• All tests run with cargo test against the Soroban mock environment — no testnet access required

Test plan

• test_happy_path_create_and_release — token flows to driver, contract balance zeroed
• test_refund_path_restores_sender_balance — sender gets tokens back
• test_dispute_resolved_to_driver — admin awards driver
• test_dispute_resolved_to_sender — admin refunds sender
• test_release_by_non_admin_rejected — panics
• test_refund_by_non_admin_rejected — panics
• test_raise_dispute_by_non_sender_rejected — panics
• test_resolve_dispute_by_non_admin_rejected — panics
• test_duplicate_delivery_id_rejected — panics
• test_release_on_already_released_rejected — panics
• test_refund_on_released_escrow_rejected — panics
• test_lifecycle_events_emitted — events vec non-empty

Closes #16

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