test(escrow): add transient TTL expiry tests

[paulinaapeh8]

• Add 15 deterministic tests in contracts/escrow/src/test/ttl_tests.rs covering all six TTL helpers (compute_expiry, store_with_ttl, read_if_live, extend_if_below_threshold, remove_transient, has_transient) for both PENDING_APPROVAL_TTL_LEDGERS (7 days) and PENDING_MIGRATION_TTL_LEDGERS (21 days)
• Tests advance env.ledger().sequence_number to prove Soroban auto-eviction: read_if_live returns Some before TTL elapses and None after
• extend_if_below_threshold: returns false for absent key, returns true and keeps entry live past original expiry after bump
• Wire ttl_tests module into contracts/escrow/src/test/mod.rs
• Add rustdoc comments to all public TTL functions and constants in ttl.rs; remove per-function #[allow(dead_code)] (crate-level allow covers it)
• Update docs/escrow/storage-ttl.md with accurate test names, helper API table, security notes, and reviewer checklist

Closes #328

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