feat: implement storage ttl extensions across state mutations

[AbuJulaybeeb]

What was done

• Abstracted extend_instance_ttl and extend_persistent_ttl wrapper functions in src/storage.rs.
• Defined ledger metric variables mapping dynamically to 30 days (BUMP_AMOUNT = 30 * DAY_IN_LEDGERS).
• Integrated persistent TTL extensions globally into the write_escrow_state helper, protecting the core EscrowState ledger persistence anytime an escrow is manipulated.
• Integrated instance TTL extensions uniformly across all major state-mutating entrypoints (create_escrow, lock_funds, release_funds, and refund) in src/contract.rs.

Why it was done

• Active contracts require consistent bumping of their Time-To-Live parameters inside the new Soroban ledger expiry model.
• By failing to perform these bump executions locally, instances carrying live escrows or administration config parameters can permanently fall off the ledger—locking buyer funds entirely.
• Exposing these wrappers abstracts away the internal manual env.storage().extend_ttl(...) configurations and standardizes our bump lengths safely across all contract states preventing logic repetition and ledger evaporation risks.

How it was verified

• cargo test executed locally resulting in completely unbroken functionality across all 26 escrow lifecycle/storage verification tests.
• Extenders strictly invoked exactly once per state mutation utilizing optimal lifetimes and triggering limits.

closes #51

[Robotron2]

Ensure the CI checks pass before the PR can be merged please. @AbuJulaybeeb