No multi-contract integration test verifying state isolation between two deployed instances #169

Problem

plan.md's Testing section explicitly asks for: "Multi-contract integration — write an integration test that deploys two contract instances and verifies that guardian state does not leak between them." No existing test deploys two VeroContract instances in the same test and asserts their storage is isolated.

Why it matters

All contract state lives in instance storage (per README.md's Storage Design section), which is scoped per-deployment — but that scoping is a Soroban platform guarantee this project has never actually exercised with a test, and instance-storage bugs (e.g. a future refactor accidentally using shared/persistent storage) wouldn't be caught by any existing single-instance test.

Suggested fix

Add a test that deploys two separate VeroContract instances (two contract_ids from env.register_contract), initializes each independently with different admins/tokens, adds a guardian to only one, and asserts is_guardian/get_reputation/get_task on the other instance shows no leakage.

Acceptance criteria

• Test deploys two instances and asserts complete state isolation across guardians, tasks, and reputation