fix(contract): make storage TTL tests assert real TTL, not record pre…

[Alanwiz00]

Summary

• Added test-only TTL utilities that use Soroban's real storage TTL APIs (get_ttl) and ledger sequence numbers, replacing timestamp-based assumptions with deterministic TTL assertions.
• Reworked the escrow and privacy TTL tests to verify actual TTL behaviour, including decay over time, extension on activity, and expiry behaviour in the local Soroban test environment.
• Clarified the distinction between business-level expiry (expires_at) and storage-layer TTL in the contract types and documentation.
• Updated the project documentation to explain why storage TTL is driven by ledger sequence numbers rather than timestamps, and documented the local sandbox's auto-restore behaviour for expired persistent entries.

Linked Issue

• Fixes TTL expiration behavior is only simulated in tests and not reflected by contract storage assertions #330

Validation

• cargo test -p rust_academy --lib passes (349/349)
• New behaviour is covered by tests
• Documentation updated to reflect the verified TTL semantics

Notes

The previous TTL tests advanced env.ledger().set_timestamp(), which does not affect Soroban storage TTL. As a result, they could pass without exercising the TTL implementation. This PR replaces those implicit assumptions with deterministic assertions based on the storage API and ledger sequence numbers, making the tests accurately reflect the contract's TTL behaviour in the Soroban test environment.

[MaryammAli]

LGTM