test: multi-contract state isolation integration test (#169)

[IfyJustin91]

Summary

Adds tests/multi_contract_isolation.rs — an integration test that deploys two independent VeroContract instances in the same Soroban test environment and asserts that instance storage is fully isolated between them.

Tests added

Test	What is verified
test_guardian_state_does_not_leak_between_instances	Guardian registered in A is is_guardian=false and get_reputation=None in B; task in A is get_task=None in B
test_state_registered_in_b_does_not_appear_in_a	Bidirectional mirror of the above
test_admin_state_is_isolated_per_instance	Each instance stores its own admin independently
test_overlapping_task_ids_are_isolated_per_instance	Same task ID and guardian address registered in both instances hold independent state
test_pause_state_is_isolated_per_instance	Pausing instance A does not affect instance B's pause state

All five tests pass:

test test_guardian_state_does_not_leak_between_instances ... ok
test test_state_registered_in_b_does_not_appear_in_a ... ok
test test_admin_state_is_isolated_per_instance ... ok
test test_overlapping_task_ids_are_isolated_per_instance ... ok
test test_pause_state_is_isolated_per_instance ... ok

test result: ok. 5 passed; 0 failed; 0 ignored

Acceptance criteria

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

Closes #169

[N-i-xx]

This needs a Closes #<issue-number> line in the description before it can merge — GrantFox links the payout to that reference. Which issue does this resolve? CI is green otherwise.