Add integration test for the escrow persistent storage TTL being extended on each deposit transaction

[Chucks1093]

Summary

The escrow's persistent storage entry must have its TTL extended on every deposit to prevent expiry during an active funding round. An integration test should confirm the TTL is at or above the minimum threshold after each deposit.

Scope

• Deploy the escrow contract on a local Soroban environment
• Execute a deposit and read the TTL of the escrow's persistent storage entry
• Assert the TTL is at or above the minimum required threshold
• Execute a second deposit and assert the TTL is reset (not accumulated)
• Assert no TTL extension occurs on a failed deposit (capacity exceeded)

Acceptance Criteria

• TTL at or above minimum after successful deposit
• TTL reset on subsequent deposits (not accumulated)
• No TTL extension on failed deposit
• Test fails if the TTL extension call is removed from the deposit function

ETA: 12 hours

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